[BEAM-4360] Migration of ReduceByKey test suite

[VaclavPlajt]

Existing tests suit for ReduceByKey Euphoria operator was adapted for current state of Beam integration. Some tests are ignored due to not yet supported features.

There are still some open points:

• Flipping test. When whole test suit runs some test may randomly fail. Assertion error shows empty test output. Current believe is that we may have a problem somewhere around ListDataSink due to a race condition. We should address that in https://issues.apache.org/jira/browse/BEAM-4366.
• Sorting of values. It is not yet clear how to implement this feature in beam.

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[VaclavPlajt]

This pull request is replaced by #seznam#1