cryptotest.py: Add a seatbelt that we're actually testing anything

Without nist-kat installed, cryptotest.py is a no-op.  Showing 'success' in
that case is unhelpful.

Sponsored by:	Dell EMC Isilon

tests/sys/opencrypto/cryptotest.py

@@ -40,6 +40,7 @@
 katdir = '/usr/local/share/nist-kat'
 
 def katg(base, glob):
+	assert os.path.exists(os.path.join(katdir, base)), "Please 'pkg install nist-kat'"
 	return iglob(os.path.join(katdir, base, glob))
 
 aesmodules = [ 'cryptosoft0', 'aesni0', 'ccr0' ]