For Kerberos testing, disable DNS lookups

For Kerberos testing, disable DNS lookups

Similar to 8dff2f224, this disables DNS lookups by the Kerberos library
to look up the KDC and the realm while the Kerberos tests are running.
In some environments, these lookups can take a long time and end up
timing out and causing tests to fail.  Further, since this isn't really
our domain, we shouldn't be sending out these DNS requests during our
tests.