Running tests of dynamic work rebalancing #17844
Description
SourceTestUtils has an exhaustive test for splitAtFraction. However, we should also add a runner-independent test that, for example, can verify that dynamic work rebalancing is actually invoked for a new BoundedSource.
An example of this test was proposed in https://github.com/GoogleCloudPlatform/DataflowJavaSDK/blob/master/sdk/src/test/java/com/google/cloud/dataflow/sdk/transforms/GroupByKeyTest.java#L435 , but was specific to the Google Cloud Dataflow service.
Imported from Jira BEAM-75. Original Jira may contain additional context.
Reported by: dhalperi.