Add Integration tests using streaming pipelines to Python 3 test suite

Integration tests using streaming pipelines for the Python SDK need to be added to the Python 3 test suite[1].
Currently, they only run for Python 2[2] on the Direct Runner as TestStream is not supported on the TestDataflowRunner.

[1] https://github.com/apache/beam/tree/master/sdks/python/test-suites/direct
[2] https://github.com/apache/beam/blob/master/sdks/python/build.gradle#L118