Fix test_multi_valued_singleton_side_input on DataflowRunner in streaming

test_multi_valued_singleton_side_input is a VR test and failed to run on DataflowRunner in streaming.

Example failure link from pantheon:
https://pantheon.corp.google.com/dataflow/jobsDetail/locations/us-central1/jobs/2018-07-25_16_16_40-5301315089771762711?project=apache-beam-testing

log:
File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Py_VR_Dataflow_PR/src/sdks/python/apache_beam/pvalue.py", line 422, in _from_runtime_iterable
ValueError: PCollection of size 2 with more than one element accessed as a singleton view. First two elements encountered are "3", "4".

We should reenable this test in beam_PostCommit_Py_VR_Dataflow after fix it.

Imported from Jira [BEAM-5025](https://issues.apache.org/jira/browse/BEAM-5025). Original Jira may contain additional context.
Reported by: markflyhigh.