Fix timing-sensitive regression test result I just created :-( --- the

Fix timing-sensitive regression test result I just created :-( --- the
DROP USER at the end of the cluster.sql test could fail, if the temp
table created in the previous session hadn't finished getting dropped.
Unluckily, I didn't see this in several repetitions of the parallel
regression tests, but it's popping up on quite a few buildfarm machines.