Unify test suite configuration structure across Py2 and Py 3 suites

Currently most of Py27 test suites are defined in https://github.com/apache/beam/blob/master/sdks/python/build.gradle while Python 3.x suites are defined in runner-specific folders, for example https://github.com/apache/beam/blob/master/sdks/python/test-suites/direct/py35/build.gradle.

This may cause confusion (where to add a new suite) and divergence of tests suites, for example Direct runner postcommit suites on Python 2 and Python 3 are defined inconsistently https://issues.apache.org/jira/browse/BEAM-7674

We should unify test structure to avoid confusion and consider reducing duplication of Gradle code if/when it makes sense to do so.