* Implement a few utility functions for tests
Most tests would require a test file hierarchy instead of operating on a
live system.
* Add unit tests for `tab::scan_path`
Tests:
* Works on a valid path
* Returns an empty Vec on an invalid directory
* Returns an empty Vec for an empty directory
I also implemented a few test helpers that may be useful for other
unit tests.
* Less spammy test logs and placate Clippy.
