Fix gitlab foss build

What does this MR do?

tmp/tests is .gitignore'd. In !24000 (merged) I force-added a tmp/tests/.gitkeep to make the file exist in the repository, but this failed to be transferred to gitlab-foss: https://gitlab.com/gitlab-org/gitlab/-/tree/master/tmp%2Ftests vs https://gitlab.com/gitlab-org/gitlab-foss/-/tree/master/tmp%2Ftests

Since we can't make changes to gitlab-foss directly any more, the best way to fix this is to change how we're doing things in gitlab.

This MR removes the .gitkeep file, getting the two repositories back into sync. It then changes the place where we put the temporary file, so it's in a directory that we know exists in the repository (tmp/ rather than tmp/tests).

Screenshots

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

Closes #210022 (comment 303073730)