Update references to the `test_file_finder` gem
What does this MR do and why?
The move had been performed at gitlab-org/ruby/gems/test_file_finder#20 (comment 1596590726).
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Edited by Rémy Coutable