Test helpers and unit tests (#26)

* 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.

Cargo.toml

@@ -47,3 +47,10 @@ debug = true
 
 [target.'cfg(unix)'.dependencies]
 fork = "0.1"
+
+[dev-dependencies]
+# cap-std = "3"
+# cap-tempfile = "3"
+fastrand = "2"
+tempfile = "3"
+test-log = "0.2"