Use api for precondition fabrication in mr_from_fork spec

What does this MR do and why?

Every single fabrication of preconditions for qa/specs/features/browser_ui/3_create/merge_request/merge_merge_request_from_fork_spec.rb is done through UI which makes the spec very slow (forked mr fabrication alone takes 1 minute on average).

This mr refactors all precondition fabrication to use api instead (creation of upstream and forked projects, commit and mr creation).

Part of: https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/7656

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