SUNRPC: Add KUnit tests for rpcsec_krb5.ko

The Kerberos RFCs provide test vectors to verify the operation of
an implementation. Introduce a KUnit test framework to exercise the
Linux kernel's implementation of Kerberos.

Start with test cases for the RFC 3961-defined n-fold function. The
sample vectors for that are found in RFC 3961 Section 10.

Run the GSS Kerberos 5 mechanism's unit tests with this command:

$ ./tools/testing/kunit/kunit.py run \
	--kunitconfig ./net/sunrpc/.kunitconfig

Tested-by: Scott Mayhew <smayhew@redhat.com>
Reviewed-by: Simo Sorce <simo@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>