Fix race condition in parallel regression tests. The new plancache test

Fix race condition in parallel regression tests.  The new plancache test
was expecting there to be no regular table named 'foo', but it turns out
the rules test transiently creates one, so that plancache would sometimes
fail.  I couldn't reproduce that in quite a few tries here, but several
buildfarm machines have shown the failure.  Fix by renaming plancache's
temp table to something nonconflicting.