Force LC_NUMERIC to C while running TAP tests.

Force LC_NUMERIC to C while running TAP tests.

We already forced LC_MESSAGES to C in order to get consistent
message output, but that isn't enough to stabilize messages
that include %f or similar formatting.

I'm a bit surprised that this hasn't come up before.  Perhaps
we ought to back-patch this change, but I'll refrain for now.

Reported-by: Bernd Helmle <mailings@oopsware.de>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/6f024eaa7885eddf5e0eb4ba1d095fbc7146519b.camel@oopsware.de