This reverts commit 56b6ef89 and instead
makes vcregress.pl parse out PROVE_FLAGS from a command line argument
when doing a TAP test, thus making it consistent with the makefile
treatment.

Discussion: https://postgr.es/m/c26a7416-2fb9-34ab-7991-618c922f896e%402ndquadrant.com

Backpatch to 9.4 like previous patch.