Add a regression test for contrib/pg_prewarm.
We had a little bit of coverage here thanks to
e2f65f425,
but not enough; notably, autoprewarm wasn't exercised at all.
Dong Wook Lee, with help from Julien Rouhaud and myself
Discussion: https://postgr.es/m/
20220629053812.mifmdrch5iuasg2s@home-desktop