Try to avoid PG::ProtocolViolation error in tests

While running our test suite, we were getting an exception sometimes:

```
Proposal Notifications In-app notifications from the proposal's author Followers should receive a notification
   Failure/Error: notification_for_user2 = Notification.find_by(user: user2)
      ActiveRecord::StatementInvalid:
        PG::ProtocolViolation: ERROR:  bind message supplies 0 parameters, but prepared statement "" requires 2
        : SELECT  "notifications".* FROM "notifications" WHERE "notifications"."user_id" = $1 LIMIT $2
   # ./spec/system/proposal_notifications_spec.rb:268
```

Sometimes we were getting a similar exception in a different test:

```
Commenting legislation questions Merged comment threads Reply on a multiple annotation thread and display it in the single annotation thread
And sometimes we were getting a different one:
   Failure/Error: annotation.comments.roots.sort_by_most_voted.limit(Legislation::Annotation::COMMENTS_PAGE_SIZE).each do |comment|
     ActionView::Template::Error:
       PG::ProtocolViolation: ERROR:  bind message supplies 0 parameters, but prepared statement "" requires 3
```

My best (wild) guess is these exceptions might take place because the
test is accessing the database and at the same time the browser
(chromedriver) process is accessing the database, with code like:

```
 find(".icon-notification").click
 notification_for_user2 = Notification.find_by(user: user2)
```

Or:

```
first(:css, ".annotator-hl").click
(...)
comment = annotation1.comments.first
click_link "Reply"
```

This behavior happened sometimes while using transactional fixtures and
a shared database connection with feature specs (before system specs
were introduced in Rails 5.1) when some queries were triggered from the
test after the browser process was started.

So we're avoiding the situation by writing the tests from the user's
point of view. This is just an attempt at fixing the issue; I don't know
whether these changes will fix it since I've only seen this exception on
Github Actions (never on my machine). Worst case scenario, we're still
improving the tests legibililty.

spec/system/comments/legislation_annotations_spec.rb

@@ -618,6 +618,8 @@ describe "Commenting legislation questions" do
       create(:legislation_annotation, draft_version: draft_version, text: "my other annotation",
                                       ranges: [{ "start" => "/p[1]", "startOffset" => 1, "end" => "/p[1]", "endOffset" => 10 }])
     end
+    let!(:comment1) { annotation1.comments.first }
+    let!(:comment2) { annotation2.comments.first }
 
     before do
       login_as user
@@ -648,15 +650,14 @@ describe "Commenting legislation questions" do
         first(:link, "0 replies").click
       end
 
-      comment = annotation1.comments.first
       click_link "Reply"
 
-      within "#js-comment-form-comment_#{comment.id}" do
+      within "#js-comment-form-comment_#{comment1.id}" do
         fill_in "Leave your comment", with: "replying in single annotation thread"
         click_button "Publish reply"
       end
 
-      within "#comment_#{comment.id}" do
+      within "#comment_#{comment1.id}" do
         expect(page).to have_content "replying in single annotation thread"
       end
 
@@ -685,17 +686,16 @@ describe "Commenting legislation questions" do
         find(".icon-expand").click
       end
 
-      comment = annotation2.comments.first
-      within("#comment_#{comment.id}") do
+      within("#comment_#{comment2.id}") do
         click_link "Reply"
       end
 
-      within "#js-comment-form-comment_#{comment.id}" do
+      within "#js-comment-form-comment_#{comment2.id}" do
         fill_in "Leave your comment", with: "replying in multiple annotation thread"
         click_button "Publish reply"
       end
 
-      within "#comment_#{comment.id}" do
+      within "#comment_#{comment2.id}" do
         expect(page).to have_content "replying in multiple annotation thread"
       end
 

spec/system/proposal_notifications_spec.rb

@@ -200,10 +200,11 @@ describe "Proposal Notifications" do
 
       find(".icon-notification").click
 
-      notification_for_user1 = Notification.find_by(user: user1)
       expect(page).to have_css ".notification", count: 1
-      expect(page).to have_content "There is one new notification on #{proposal.title}"
-      expect(page).to have_xpath "//a[@href='#{notification_path(notification_for_user1)}']"
+
+      click_link text: "There is one new notification on #{proposal.title}"
+
+      expect(page).to have_current_path(proposal_path(proposal))
 
       logout
       login_as user2
@@ -211,10 +212,11 @@ describe "Proposal Notifications" do
 
       find(".icon-notification").click
 
-      notification_for_user2 = Notification.find_by(user: user2)
       expect(page).to have_css ".notification", count: 1
-      expect(page).to have_content "There is one new notification on #{proposal.title}"
-      expect(page).to have_xpath "//a[@href='#{notification_path(notification_for_user2)}']"
+
+      click_link text: "There is one new notification on #{proposal.title}"
+
+      expect(page).to have_current_path(proposal_path(proposal))
 
       logout
       login_as user3
@@ -251,10 +253,11 @@ describe "Proposal Notifications" do
 
       find(".icon-notification").click
 
-      notification_for_user1 = Notification.find_by(user: user1)
       expect(page).to have_css ".notification", count: 1
-      expect(page).to have_content "There is one new notification on #{proposal.title}"
-      expect(page).to have_xpath "//a[@href='#{notification_path(notification_for_user1)}']"
+
+      click_link text: "There is one new notification on #{proposal.title}"
+
+      expect(page).to have_current_path(proposal_path(proposal))
 
       logout
       login_as user2.reload
@@ -262,10 +265,11 @@ describe "Proposal Notifications" do
 
       find(".icon-notification").click
 
-      notification_for_user2 = Notification.find_by(user: user2)
       expect(page).to have_css ".notification", count: 1
-      expect(page).to have_content "There is one new notification on #{proposal.title}"
-      expect(page).to have_xpath "//a[@href='#{notification_path(notification_for_user2)}']"
+
+      click_link text: "There is one new notification on #{proposal.title}"
+
+      expect(page).to have_current_path(proposal_path(proposal))
 
       logout
       login_as user3.reload
@@ -301,10 +305,8 @@ describe "Proposal Notifications" do
 
       find(".icon-notification").click
 
-      notification_for_user = Notification.find_by(user: user)
       expect(page).to have_css ".notification", count: 1
       expect(page).to have_content "This resource is not available anymore"
-      expect(page).not_to have_xpath "//a[@href='#{notification_path(notification_for_user)}']"
     end
 
     scenario "Proposal retired by author", :js do