lib-gpg: allow tests with GPGSM or GPGSSH prereq first

When the 'GPG' prereq is lazily tested, `mkdir "$GNUPGHOME"` could
fail if the "$GNUPGHOME" directory already exists. This can happen if
the 'GPGSM' or the 'GPGSSH' prereq has been lazily tested before as they
already create "$GNUPGHOME".

To allow the GPGSM or the GPGSSH prereq to appear before the GPG prereq
in some test scripts, let's refactor the creation and setup of the
"$GNUPGHOME"` directory in a new prepare_gnupghome() function that uses
`mkdir -p "$GNUPGHOME"`.

This will be useful in a following commit.

Unfortunately the new prepare_gnupghome() function cannot be used when
lazily testing the GPG2 prereq, because that would expose existing,
hidden bugs in "t1016-compatObjectFormat.sh", so let's just document
that with a NEEDSWORK comment.

Helped-by: Todd Zullinger <tmz@pobox.com>
Helped-by: Collin Funk <collin.funk1@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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