gh-126014: test_makefile_test_folders: Ignore basically-empty directories (#140466)

The code in test_makefile was attempting to ignore any
non-interesting files, but missed some corners:

1. There is never a *file* called `__pycache__`.
2. A directory containing only a `__pycache__` subdirectory should be
   ignored.
3. A directory containing only hidden files should be ignored.

Simplify this all into a couple of filters that let us check for empty
lists.