[Batch Fetching] Fix and re-enable test to ensure batch fetches aren't started too often.

[appleguy]

I've found from some recent work that the overall perf cost of an extra batch / continuation call has substantial impacts on user metrics, including time to display the first content (competition between image loading and data loading) as well as the parsing / layout of the response slowing down content interaction and rendering.

As such, we should strive to make this test more robust and ensure it is working correctly. I believe it may actually be generating too many calls in some cases.

#649