Make new regression test case parallel-safe, and improve its output.

Make new regression test case parallel-safe, and improve its output.

The test case added by commit 1f6d515a6 fails on buildfarm members that
have force_parallel_mode turned on, because we currently don't report sort
performance details from worker processes back to the master.  To fix that,
just make the test table be temp rather than regular; that's a good idea
anyway to forestall any possible interference from auto-analyze.
(The restriction that workers can't access temp tables might go away
someday, but almost certainly not before the other thing gets fixed.)

Also, improve the test so that we retain as much as possible of the
EXPLAIN ANALYZE output.  This aids debugging failures, and might also
expose problems that the preceding version masked.

Discussion: http://postgr.es/m/CADE5jYLuugnEEUsyW6Q_4mZFYTxHxaVCQmGAsF0yiY8ZDggi-w@mail.gmail.com