QA: Fix failure in merge_merge_request_from_fork_spec.rb

What does this MR do?

Handle merging with pipeline in-progress

The end-to-end tests expected to be able to merge immediately. This isn't the case if there's a pipeline in progress - the default is to merge when the pipeline passes. That's not ok for typical test scenarios and was failing pipelines, e.g. https://gitlab.com/gitlab-org/quality/nightly/-/jobs/99102829

This MR detects if there are options to merge immediately or after the pipeline passes and it selects merge immediately

What are the relevant issue numbers?

#51657 (closed)

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

Closes #51657 (closed)