On the documentation the examples for the usage of isolate_tests.py are shown with single files, and it's currently not working. It only works for folders or wildcards that return more than one file, since that's how os.walk works within a loop for that cases.
Proposed an simple and easy fix.
I extracted the core functionality for extracting tests from files, and made another function called `extract_and_write`
If the program receives a single file the function `extract_and_write` is called once, it even works for `docs` when specified.
If the program receives a path or a wildcard, works as used to.
