Wait for push before trying to create a new MR

What does this MR do?

Make Resource::MergeRequest wait during fabrication for the push to happen before trying to open a merge request.

This would sometimes cause tests to fail.

This isn't the first time this sort of thing was needed. This suggest a more general fix should have a higher priority: https://gitlab.com/gitlab-org/gitlab-ce/issues/53308

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/54302#note_124108092

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer