Simplify MWPS spec to avoid merge conflict error

What does this MR do and why?

Describe in detail what your merge request does and why.

merge_when_pipeline_succeeds_spec.rb sometimes run into these failures #359999 (closed) & #351965 (closed) because auto merge got aborted when a merge conflict was detected.

Our current merge request resource is smart enough to create a new source branch at the creation of the MR so we don't need to do so after creating the MR. Also with only:merge_request rule, a pipeline is automatically created as soon as the MR is pushed with a new change, we don't need to commit another new change to the MR to avoid running into merge conflicts.

I'm simplifying the test (not pushing extra branch nor extra commit) to avoid this. Should also help improve test execution time a little, too.

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Related to #359999 (closed) & #351965 (closed)