Add more expectations in SDG Management search spec

We've had some tests fail after this test was executed in our CI, and
one possible reason could be that sometimes this test finished before
all its requests had finished. This could be the case with the following
code:

```
visit sdg_management_proposals_path(filter: "pending_sdg_review")

click_button "Search"

expect(page).to have_css "li.is-active h2", exact_text: "Pending"
```

Before clicking the "Search" button, the expectation is already true, so
there's a chance that the expectation is evaluated as true before the
request has finished. That might result in requests and session data
leaking between tests.

So we're adding more expectations in order to make sure that the
requests have finished before evaluating the expectations associated to
them.

spec/system/sdg_management/relations_spec.rb

@@ -205,22 +205,41 @@ describe "SDG Relations" do
       end
 
       scenario "search within current tab" do
+        create(:proposal, title: "Proposal pending review")
+        create(:sdg_review, relatable: create(:proposal, title: "Proposal already reviewed"))
+
         visit sdg_management_proposals_path(filter: "pending_sdg_review")
 
+        expect(page).to have_content "Proposal pending review"
+        expect(page).not_to have_content "Proposal already reviewed"
+
+        fill_in "search", with: "non existent"
         click_button "Search"
 
+        expect(page).not_to have_content "Proposal pending review"
         expect(page).to have_css "li.is-active h2", exact_text: "Pending"
 
         visit sdg_management_proposals_path(filter: "sdg_reviewed")
 
+        expect(page).not_to have_content "Proposal pending review"
+        expect(page).to have_content "Proposal already reviewed"
+
+        fill_in "search", with: "non existent"
         click_button "Search"
 
+        expect(page).not_to have_content "Proposal already reviewed"
         expect(page).to have_css "li.is-active h2", exact_text: "Marked as reviewed"
 
         visit sdg_management_proposals_path(filter: "all")
 
+        expect(page).to have_content "Proposal pending review"
+        expect(page).to have_content "Proposal already reviewed"
+
+        fill_in "search", with: "non existent"
         click_button "Search"
 
+        expect(page).not_to have_content "Proposal pending review"
+        expect(page).not_to have_content "Proposal already reviewed"
         expect(page).to have_css "li.is-active h2", exact_text: "All"
       end