Bazel support: read test specs from TESTBRIDGE_TEST_ONLY

[jfirebaugh]

Description Similarly to how Catch2 reads the BAZEL_TEST and XML_OUTPUT_FILE environment variables to support JUnit XML for Bazel, it should read test specs from the TESTBRIDGE_TEST_ONLY environment variable, which will be set when running bazel test with a --test_filter argument.

[horenmar]

That's the worst env variable name I've ever seen.