Increase testing coverage of "UI for disabling group/project email notifications"

The following discussion from gitlab-ce!30961 should be addressed:

• @pslaughter started a discussion: (+2 comments)

  test: I see we don't have a spec for group/settings. Do we need this?

  It'd also be nice to have a spec that showed when the setting is checked and saved, it stays checked on page refresh and the BE value is udpated 🤷