tests: squelch noise from GPG machinery set-up

tests: squelch noise from GPG machinery set-up

It is distracting to let the GPG message while setting up the test
gpghome leak into the test output, especially without running these
tests with "-v" option.

The splitting of RFC1991 prerequiste part is about future-proofing.
When we want to define other kinds of specific prerequisites in the
future, we'd prefer to see it done separately from the basic set-up
code.

Signed-off-by: Junio C Hamano <gitster@pobox.com>