Create better git UI workflow tests

Seems that we failed to notice that git UI workflows are missing since CRW 2.3.

This suggests we're not testing the git workflows within Theia (or not testing them enough!), so I'd like to propose we have new happy path tests that include verifying git UI works and isn't missing. Once users get used to something, having it vanish unexpectedly is not ideal, and we want to avoid regressions like CRW-1581 in future.