Flakey TestSpaceQuotasWithSnapshots

I've started seeing some intermittent unit test failures in this class.

After digging in, it seems like the expected usage size is ~10KB different than what we want. The tests wait to see the region size reports (physical size of the table) to reach a certain range, but it seems like sometimes there is a small change after the test already decided what the final "size" was.

Should be able to stabilize the test by waiting for the region size report to be constant over a few iterations.