Exercise Dataflow runner integration tests in a postcommit suite for Python 3.5 and 3.6

https://github.com/apache/beam/pull/7756 adds a test suite with a single test. Once merged, we need to extend the suite to all available integration tests. If some tests are not passing, we can track any remaining parallelizable work in separate JIRAs.  

cc: Juta

cc: markflyhigh