Speed up "brin" regression test a little bit.

In the large DO block, collect row TIDs into array variables instead of
creating and dropping a pile of temporary tables.  In a normal build,
this reduces the brin test script's runtime from about 1.1 sec to 0.4 sec
on my workstation.  That's not all that exciting perhaps, but in a
CLOBBER_CACHE_ALWAYS test build, the runtime drops from 20 min to 17 min,
which is a little more useful.  In combination with some other changes
I plan to propose, this will help provide a noticeable reduction in
cycle time for CLOBBER_CACHE_ALWAYS buildfarm critters.