FIX: flaky flags system spec (#29039)

Because of unreliability, the spec was temporarily disabled. However, it is ensuring that the custom flags system is working correctly. Therefore it would be great to enable it again.

I made a few fixes to try to mitigate this situation:
- Reduced amount of Redis calls;
- When deleting, ensure that the modal is closed before checking the result;
- Moved duplicated name tests to a separate block;
- Increased wait time to 3 times the default because I noticed that sometimes it gets stuck for a moment. Most of the time it is fast, but sometimes when I run tests in a loop 50 times I see slowness.

spec/system/page_objects/pages/admin_flag_form.rb

@@ -23,6 +23,11 @@ module PageObjects
 
       def click_save
         form.submit
+        expect(page).to have_no_css(
+          ".admin-config.flags.new",
+          wait: Capybara.default_max_wait_time * 3,
+        )
+        expect(page).to have_css(".admin-flag-item__name", wait: Capybara.default_max_wait_time * 3)
       end
 
       def form

spec/system/page_objects/pages/admin_flags.rb

@@ -60,7 +60,7 @@ module PageObjects
       end
 
       def has_no_flag?(flag)
-        has_no_css?(".admin-flag-item.#{flag}")
+        has_no_css?(".admin-flag-item.#{flag}", wait: Capybara.default_max_wait_time * 3)
       end
 
       def has_saved_flag?(key)
@@ -103,6 +103,7 @@ module PageObjects
 
       def confirm_delete
         find(".dialog-footer .btn-primary").click
+        expect(page).to have_no_css(".dialog-body", wait: Capybara.default_max_wait_time * 3)
         self
       end