Test (some of) GmailElementReplacer tests in mock environment

[rrrooommmaaa]

For this we would need to extend our mock gmail endpoints, like mail/u/0/#inbox/FMfcgzGrbHrBdFGBXqpFZvSkcQpKkvrM
to return a page similar to what gmail once returned

Originally posted by @rrrooommmaaa in #4807 (comment)