Do not convert --test_filter a.py to a::py

[raylu]

Currently, if you try to filter a test target to a file with --test_filter some_test.py,
the shim converts it to some_test::py and deselects all test cases.

---

To test this change, in our workspace's MODULE.bazel, I added

git_override(
    module_name = "caseyduquettesc_rules_python_pytest",
    remote = "https://github.com/Benchling-Open-Source/rules_python_pytest.git",
    commit = "727a5037ec5b743ac2ffa52bbdd6968b427954be",
)

and ran
$ bazel test //tests/unit/some_target --test_filter some_test.py --test_output streamed
and confirmed that only the tests in some_test.py ran

[caseyduquettesc]

Yikes, thanks!

[raylu]

wow, so fast! released already too! thanks!