Adding merge trains license_check to specs

What does this MR do and why?

There was some confusion if license checks were being incorporate into merge train checks. There is, but the specs don't easily test for this.

This MR is just to make the spec more clear about these license conditions for merge checks

References

https://gitlab.com/gitlab-org/gitlab/-/issues/567853#note_2836101797

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Before	After

How to set up and validate locally

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Related to #567853