gpg: Add hack to --quick-gen-key to create Curve25519 keys.

* g10/keygen.c (quick_generate_keypair): Add special algo string
"test-default".

Well, this is a hack to quickly create keys with the algorithms we
will eventually use as defaults. Usage:

gpg -v --quick-gen-key --passphrase '' --batch USERID test-default

• Signed-off-by: Werner Koch <wk@gnupg.org>