Use faker unique method to make sure filepath is always different

What does this MR do?

To address test flakiness #323701 (closed). Having generate a random word with suffix .txt there is still a chance this word is same as the previous one causing

Fabrication of QA::Resource::Repository::Commit using the API failed (400) with `{"message":"A file with this name already exists"}`.

Explicitly calling unique to ensure uniqueness between back to back file commits.

Does this MR meet the acceptance criteria?

Conformity

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Related to #323701 (closed)