tests: increase the verbosity of the GPG-related prereqs

tests: increase the verbosity of the GPG-related prereqs

Especially when debugging a test failure that can only be reproduced in
the CI build (e.g. when the developer has no access to a macOS machine
other than running the tests on a macOS build agent), output should not
be suppressed.

In the instance of `hi/gpg-prefer-check-signature`, where one
GPG-related test failed for no apparent reason, the entire output of
`gpg` and `gpgsm` was suppressed, even in verbose mode, leaving
interested readers no clue what was going wrong.

Let's fix this by no longer redirecting the output not to `/dev/null`.
This is now possible because the affected prereqs were turned into lazy
ones (and are therefore evaluated via `test_eval_` which respects the
`--verbose` option).

Note that we _still_ redirect `stdout` to `/dev/null` for those commands
that sign their `stdin`, as the output would be binary (and useless
anyway, because the reader would not have anything against which to
compare the output).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>