[Random spec failure] Move `merge_immediately_with_pipeline_spec.rb` to specs/javascript

This spec causes a random failure of any spec that goes after it!

We should move spec/features/merge_requests/merge_immediately_with_pipeline_spec.rb to 'spec/javascript' or do something else.

Discussion in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1475#note_25970075 :

find('.dropdown-toggle').click

click_link 'Merge Immediately'

expect(find('.js-merge-when-pipeline-succeeds-button')).to have_content('Merge in progress')

Text "Merge in progress" is filling out by JS on click!!! not as success callback of AJAX request. The expectation block finds the text and the runtime goes to the next spec at the same time the AJAX query is performing in parallel. Before we run next spec we clean database and the AJAX request of course fails. That's a reason of this weird failure. Note! Fails the next spec, not this one!!!

PS @iamphill If we don't test server response maybe we should move this spec to spec/javascript? I don't think it has any value...