Follow-up from "Replace the 'profile/active_tab.feature' spinach test with an rspec analog"

The following discussion from !14239 (merged) should be addressed:

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

  Is this the first conversion dealing with these ensure_active_main_tab checks? We probably want a custom matcher so that if we ever update navigation again we're not changing 50+ selectors in specs.

/cc @rspeicher @blackst0ne