Allow specifying output file basename instead of just directory

This is optional, and doesn't impact existing users as preexisting
behavior when users specify an output directory is unchanged.

This makes tests not need to copy large files as part of their
execution, as they can ask files to be written to explicit
locations.

Signed-off-by: Will Beason <willbeason@gmail.com>

.gitignore

@@ -13,4 +13,3 @@ uv.lock
 # Python build and test output
 __pycache__/
 /test/test_output/
-/test/test_output.parquet/