Resolve transient failure in Help page spec

Sometimes due to a slow request to load the version check image, the
placeholder image data was still being seen, resulting in this failure:

    expected "data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==" to end with "/version-check-url"

Now we check the `data-src` attribute, which allows us to remove the
`:js` metadata.

This commit also removes a redundant test, which was just ensuring that
the selector we were already using in the other test is visible. If this
test were failing, the other one would always fail too, so it was
pointless.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/55372

spec/features/help_pages_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe 'Help Pages' do
@@ -52,7 +54,7 @@ describe 'Help Pages' do
     end
   end
 
-  context 'in a production environment with version check enabled', :js do
+  context 'in a production environment with version check enabled' do
     before do
       stub_application_setting(version_check_enabled: true)
 
@@ -64,12 +66,9 @@ describe 'Help Pages' do
     end
 
     it 'has a version check image' do
-      expect(find('.js-version-status-badge', visible: false)['src']).to end_with('/version-check-url')
-    end
-
-    it 'hides the version check image if the image request fails' do
-      # We use '--load-images=yes' with poltergeist so the image fails to load
-      expect(page).to have_selector('.js-version-status-badge', visible: false)
+      # Check `data-src` due to lazy image loading
+      expect(find('.js-version-status-badge', visible: false)['data-src'])
+        .to end_with('/version-check-url')
     end
   end