Work around need for $HOME/.gnupg directory.

If $HOME/.gnupg does not exist, gpg fails during initial key generation
with:
    gpg: keyblock resource `$HOME/.gnupg/secring.gpg': file open error
    gpg: keyblock resource `$HOME/.gnupg/pubring.gpg': file open error

gpg returns a non-zero exist code, stopping key generation and
ultimately, causing test failures.

Per:
    http://www.gossamer-threads.com/lists/gnupg/users/62155
--keyring /dev/null and --secret-keyring /dev/null work around this.