Modernize `fs_test.py` to use type hints and `assert` statements (#9858)
Modernize `fs_test.py` to use type hints and `assert` statements (#9858)

The real goal is to get rid of `FileSystemProjectTree`. This is prework to make the tests easier to work with and to follow our style guide.

[ci skip-rust-tests]
[ci skip-jvm-tests]
pantsbuild/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants,jsirois/pants,benjyw/pants,benjyw/pants,benjyw/pants,jsirois/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,jsirois/pants