TAP tests still sensitive to various PG* environment variables

Hi all,

While looking at a separate issue, I have noticed that TestLib.pm is
lagging behind in terms of environment variables it had better mask to
avoid failures:
https://www.postgresql.org/message-id/YLXjFOV3teAPirmS@paquier.xyz

Once I began playing with the variables not covered yet, and tested
fancy cases with junk values, I have been able to see various failures
in the TAP tests, mainly with authentication and SSL.

Attached is a patch to strengthen all that, which I think we'd better
backpatch.

Any objections to that?
--
Michael