PKCS#7: Add MODULE_LICENSE() to test module

Add a MODULE_LICENSE() line to the PKCS#7 test key module to fix this
warning:

	WARNING: modpost: missing MODULE_LICENSE() in
	crypto/asymmetric_keys/pkcs7_test_key.o

Whilst we're at it, also add a module description.

Reported-by: James Morris <jmorris@namei.org>
Signed-off-by: David Howells <dhowells@redhat.com>