SUNRPC: Let Kunit tests run with some enctypes compiled out

Allow the new GSS Kerberos encryption type test suites to run
outside of the kunit infrastructure. Replace the assertion that
fires when lookup_enctype() so that the case is skipped instead of
failing outright.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>