Test creating a merge request when pipelines are enabled

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Description

As https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7568 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7664 showed we lack some important test coverage for Projects::MergeRequestsController when creating a merge request and pipelines are enabled for project.

Proposal

Test it all!

/cc @DouweM @smcgivern @ayufan