[Bug]: Uncomment TFX-BSL tests once tfx-bsl supports protobuf 4.x

### What happened?

https://github.com/tensorflow/tfx-bsl/blob/c5306a1682b1c71240f92dc83682344e482a7efd/setup.py#L175 tfx-bsl doesn't support protobuf 4.x.x. So the tests are commented at https://github.com/apache/beam/pull/24599

Uncomment the tests once tfx-bsl supports protobuf 4.x.x.

### Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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