CI and test improvements

I'm "joining" a bunch of unresolved threads hoping to present them better since
they're related and I'm maintaining them together anyway.

https://www.postgresql.org/message-id/flat/20220219234148.GC9008%40telsasoft.com
 - set TESTDIR from perl rather than Makefile
https://www.postgresql.org/message-id/flat/20220522232606.GZ19626%40telsasoft.com
 - ccache, MSVC, and meson
https://www.postgresql.org/message-id/20220416144454.GX26620%40telsasoft.com
 - Re: convert libpq uri-regress tests to tap test
https://www.postgresql.org/message-id/CA%2BhUKGLneD%2Bq%2BE7upHGwn41KGvbxhsKbJ%2BM-y9nvv7_Xjv8Qog%40mail.gmail.com
 - Re: A test for replay of regression tests
https://www.postgresql.org/message-id/flat/20220409021853.GP24419@telsasoft.com
 - cfbot requests

See the commit messages for more thread references.

I'm anticipating the need to further re-arrange the patch set - it's not clear
which patches should go first.  Maybe some patches should be dropped in favour
of the meson project.  I guess some patches will have to be re-implemented with
meson (msvc warnings).

I think there was some confusion about the vcregress "alltaptests" target.
I said that it's okay to add it and make cirrus use it (and that the buildfarm
could use it too).  Andrew responded that the buildfarm wants to run different
tests separately.  But Andres seems to have interpretted that as an objection
to the addition of an "alltaptests" target, which I think isn't what's
intended - it's fine if the buildfarm prefers not to use it.

Maybe the improvements to vcregress should go into v15 ?  CI should run all the
tests (which also serves to document *how* to run all the tests, even if there
isn't a literal check-world target).