Insert temporary debugging output in regression tests.

We're seeing occasional instability in the plans generated for
parallel queries on the "a_star" table hierarchy.  This suggests
that something is changing the planner's stats for those tables,
but that should not be happening within a regression test run.
To try to gather some information about what's happening, insert
additional queries to check the basic page/tuple counts for these
tables, as well as whether any vacuums or analyzes have happened
on them.  (We expect that only the database-wide VACUUM in
sanity_check.sql will have touched them.)

I added the probes not only in select_parallel.sql itself, but
also in stats.sql, bearing in mind that the stats collector's
lag may prevent the initial query from reporting current truth.
If any extra vacuum/analyze has happened, the recheck in stats.sql
definitely ought to see it.

This commit can be reverted once we figure out what's going on.

Per suggestion from David Rowley, though I changed the queries around.

Discussion: https://postgr.es/m/CA+hUKG+0CxrKRWRMf5ymN3gm+BECHna2B-q1w8onKBep4HasUw@mail.gmail.com

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