Tests that UnboundedSources are executed correctly

Specifically, develop a set of RunnableOnService tests that validate runner behavior when executing an Unbounded Source. Validations should include behaviors such as finalizeCheckpoint being called at most once, etc.

Imported from Jira [BEAM-1654](https://issues.apache.org/jira/browse/BEAM-1654). Original Jira may contain additional context.
Reported by: bchambers.

Start by locating the existing RunnableOnService tests and the runner entry point for executing an Unbounded Source. Trace checkpoint finalization behavior, then add coverage for finalizeCheckpoint being called at most once and the other runner behaviors described in the issue. Done means the test suite validates these unbounded-source execution guarantees.

Written by the indexing model from the issue text.