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.