Support setting test filter via --test_filter

[snowp]

For a more ergonomic Bazel experience, it would be nice if the test runner would propagate the values of --test_filter to the underlying test runner. Per https://bazel.build/reference/test-encyclopedia seems like this just means reading TESTBRIDGE_TEST_ONLY and passing the values along

[UebelAndre]

Does the info at #417 help with what you're looking to do? If there's an easier way to tell tests that use use_libtest_harness to only run a subset of tests I'd be happy to learn about how to do that and provide review support for a pull-request 😃