The output of the function changes whenever previous (or, as in this
case, concurrent) tests leave a table in place. That causes unneeded
churn.

This should fix failures due to the tests added bfe16d1a, like on
lapwing, caused by the tsrf test running concurrently with misc. Those
could also have been addressed by using temp tables, but that test has
annoyed me before.

Discussion: <27626.1473729905@sss.pgh.pa.us>