Fix file commit error for system cancels merge train spec

What does this MR do?

To address API error Fabrication of QA::Resource::Repository::Commit using the API failed (400) with {"message":"A file with this name already exists"} #321464 (closed).

• In this solution, I explicitly created the file in master first.
• Make the file name to be a random string instead of hardcoded string.
• Small refactor to optimize the flow of the test.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Related to #321464 (closed)