Abstract eventually_matcher.rb so it can be used in feature specs

Follow-up from !87032 (comment 940357880)

In QA E2E specs we have eventually_ matchers that can be used for tests that need waiting. See https://docs.gitlab.com/ee/development/testing_guide/end_to_end/best_practices.html#use-eventually_-matchers-for-expectations-that-require-waiting

These matchers could be useful in feature specs for things like debouncing. If we move these matchers so they can be used in QA E2E specs and feature specs that could be helpful.