Creates a source out of the stageUnderTest
that will inform the probe
of operator events and callbacks by sending it the various messages found under
GraphStageMessages.
Creates a source out of the stageUnderTest
that will inform the probe
of operator events and callbacks by sending it the various messages found under
GraphStageMessages.
This allows for creation of a "normal" stream starting with the source while still being able to assert internal events.