Bazel support: TEST_TOTAL_SHARDS / TEST_SHARD_INDEX / TEST_SHARD_STATUS_FILE

[jfirebaugh]

Description Similarly to how Catch2 reads the BAZEL_TEST and XML_OUTPUT_FILE environment variables to support JUnit XML for Bazel, it should configure sharding based on the TEST_TOTAL_SHARDS and TEST_SHARD_INDEX environment variables, and indicate support for sharding by touching the file named in the TEST_SHARD_STATUS_FILE environment variable.

https://bazel.build/reference/test-encyclopedia#test-sharding