A way to register additional test targets and support .py only tests. (#3590)

* A way to register additional test targets.

* Support specifying tests with extension.

* Ensure TEST_OVERRIDE is backwards compatible.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci



* Ensure regex is non greedy.

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>