Make `dslx_test` capable of obeying Bazel sharding protocol

See the description of `TEST_SHARD_INDEX` here: https://docs.bazel.build/versions/2.2.0/test-encyclopedia.html#role-of-the-test-runner

For long running tests or many test cases, sharding is useful, and it's reasonably simple to implement Bazel's required protocol. This will allow "shard_count = X" on `dslx_test` build rules.