Dynamic Element Validation Iteration: Improve waiting

The following discussion from gitlab-ee!9982 should be addressed:

• @rymai started a discussion: (+1 comment)

  @ddavison We still have login_page did not appear on QA::Page::Main::Login as expected failures in some jobs (https://gitlab.com/gitlab-org/gitlab-qa/pipelines/53818875).

  I'm worried we're going to introduce some flaky failures. Do we need to wait even more (it's weird because we should already be waiting the same time as before)?

Currently we are only waiting (in validatable.rb) using has_element?. this should be replaced by our own waiting methods that have built-in explicit waiting.