PGP: Provide a key type for testing PGP signatures

Provide a key type for testing the PGP signature parser.  It is given a
non-detached PGP message as payload:

	keyctl padd pgp_test a @s <content.txt.gpg

A suitable message can be generated like this:

	echo "This is a test attached-signed content" >content.txt
	gpg --compress-algo=none -s content.txt

Signed-off-by: David Howells <dhowells@redhat.com>