Re-write sidebar_reviewers_spec.js | Follow-up from "Track MR approvals from the Review Drawer"

The following discussion from !214789 (merged) should be addressed:

• @ntepluhina started a discussion: (+4 comments)

  suggestion: we are setting component state here via modifying component internals. Looks like this is an Apollo query, could we instead mock issuable query here?

  well, we should not be providing data in the tests, and our testing guidelines suggest against this. Honestly, the whole test suite is written in a questionable way, with calling internal component methods via wrapper.vm and I would suggest this is a good opportunity to rewrite it since current implementation isn't really testing the user-facing behavior.