Need an UnboundedSourceFromHell to test runner's respect of its implied contract #18008
Description
UnboundedSource has pretty specific requirements of its runner (as well as its derived classes). We really need a systematic test of all of this. Inspiration is BEAM-306 which only showed up when I tried to run an actual pipeline.
Imported from Jira BEAM-309. Original Jira may contain additional context.
Reported by: mshields822.