Try to avoid exception after flaggable tests

We had one exception running test suite [1]:

```
  1) Commenting legislation questions Merged comment threads View
comments of annotations in an included range
     Failure/Error: count: annotation.comments.roots.count) %>

     ActionView::Template::Error:
       PG::ProtocolViolation: ERROR:  bind message supplies 0
parameters, but prepared statement "" requires 2
       : SELECT COUNT(*) FROM "comments" WHERE "comments"."hidden_at" IS
NULL AND "comments"."commentable_id" = $1 AND
"comments"."commentable_type" = $2 AND "comments"."ancestry" IS NULL
```

Debugging shows this test was run right after the flaggable specs.
There's a chance these exceptions take place because the test is
accessing the database after the browser (chromedriver) process has
already accessed the database.

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 encouraging
good practices of system tests: test things from the user's point of
view.

[1] https://github.com/consul/consul/runs/1856245992

spec/shared/system/flaggable.rb

@@ -34,7 +34,12 @@ shared_examples "flaggable" do |factory_name, admin: false|
       expect(page).not_to have_link "Flag as inappropriate", visible: :all
     end
 
-    expect(Flag.flagged?(user, flaggable)).to be
+    visit path
+
+    within "##{dom_id(flaggable)} .flag-content" do
+      expect(page).to have_link "Unflag", visible: :hidden
+      expect(page).not_to have_link "Flag as inappropriate", visible: :all
+    end
   end
 
   scenario "Unflagging", :js do
@@ -54,7 +59,12 @@ shared_examples "flaggable" do |factory_name, admin: false|
       expect(page).not_to have_link "Unflag", visible: :all
     end
 
-    expect(Flag.flagged?(user, flaggable)).not_to be
+    visit path
+
+    within "##{dom_id(flaggable)} .flag-content" do
+      expect(page).to have_link "Flag as inappropriate", visible: :hidden
+      expect(page).not_to have_link "Unflag", visible: :all
+    end
   end
 
   scenario "Flagging and unflagging", :js do
@@ -66,7 +76,6 @@ shared_examples "flaggable" do |factory_name, admin: false|
       click_link "Flag as inappropriate"
 
       expect(page).to have_css ".flag-active"
-      expect(Flag.flagged?(user, flaggable)).to be
 
       find(".icon-flag").click
       click_link "Unflag"
@@ -74,7 +83,12 @@ shared_examples "flaggable" do |factory_name, admin: false|
       expect(page).not_to have_css ".flag-active"
     end
 
-    expect(Flag.flagged?(user, flaggable)).not_to be
+    visit path
+
+    within "##{dom_id(flaggable)} .flag-content" do
+      expect(page).to have_link "Flag as inappropriate", visible: :hidden
+      expect(page).not_to have_link "Unflag", visible: :all
+    end
   end
 
   scenario "Flagging a comment with a child does not update its children", :js do