Allow individual TAP tests to be run via PROVE_TESTS

Add a new optional Makefile variable PROVE_TESTS that, if passed as a
space-separated list of paths relative to the Makefile invoking
$(prove_check) or $(prove_installcheck), runs just those tests instead
of t/*.pl .

From: Craig Ringer <craig@2ndquadrant.com>
Reviewed-by: Michael Paquier <michael.paquier@gmail.com>