Add tests for sticky merge requests tab

@iamphill wrote tests for the sticky merge request tabs but later we decided to remove them because there was a problem with the tests with a sticky layout. We should enable those tests because right now our actual page and test environment is different.

See the original discussion here https://gitlab.com/gitlab-org/gitlab-ce/commit/517895da4c1ca6201f952e443a579e4f2845e6e0