pg_regress.c also sensitive to various PG* environment variables

Hi all,

Following up with the recent thread that dealt with the same $subject
for the TAP tests, I have gone through pg_regress.c:
https://www.postgresql.org/message-id/YLbjjRpucIeZ78VQ@paquier.xyz

The list of environment variables that had better be reset when using
a temporary instance is very close to TestLib.pm, leading to the
attached.  Please note that that the list of unsetted parameters has
been reorganized to be consistent with the TAP tests, and that I have
added comments referring one and the other.

Thoughts?
--
Michael