GBK Python streaming load tests are too slow

The following GBK streaming test cases take too long on Dataflow:

1) 2GB of 10B records

2) 2GB of 100B records

4) fanout 4 times with 2GB 10-byte records total

5) fanout 8 times with 2GB 10-byte records total

Each of them needs at least 1 hour to execute, which is way too long for one Jenkins job.

Job's definition: https://github.com/apache/beam/blob/master/.test-infra/jenkins/job_LoadTests_GBK_Python.groovy

Test pipeline: https://github.com/apache/beam/blob/master/sdks/python/apache_beam/testing/load_tests/group_by_key_test.py

It is probable that those cases are too extreme. The first two cases involve grouping 20M unique keys, which is a stressful operation. A solution might be to overhaul the cases so that they would be less complex.

Both the current production Dataflow runner and the new Dataflow Runner V2 were tested.