Documenting PROVE_TESTS in section of TAP tests

Hi all,

The section of the documentation dedicated to TAP tests mentions
PROVE_FLAGS:
https://www.postgresql.org/docs/devel/static/regress-tap.html

I think that it would be a good idea to mention PROVE_TESTS as well.  I
personally use and abuse of it, and documenting it instead of keeping it
hidden in Makefile.global.in is good for new developers and testers.

Proposal of patch attached.

Thanks,
--
Michael