Extend ProcessPassthrough testing

What does this MR do?

While investigating Fix Remote Custom Rulesets (nodejs-scan!166 - merged) • Craig Smith • 17.2 I noticed that ProcessPassthroughs sometimes returns a path to a directory, and sometimes returns a path to a file. This behavior didn't seem to be unit tested so this MR adds that testing, both to assert the behavior and to document it.

What are the relevant issue numbers?

Test and fix all remote SAST remote configurati... (gitlab-org/gitlab#458845 - closed) • Craig Smith • 17.2

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests updated/added for this feature/bug
• Job definition updated, if necessary
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer