s4-auth-tests: Fix test_kerberos with MIT krb

s4-auth-tests: Fix test_kerberos with MIT krb

When a keytab of type MEMORY is used, the MIT kerberos krb5_kt_add_entry()
library function adds a keytab entry to the beginning of the keytab table,
instead of the end.

This adds a MIT kerberos conditional to reverse iterate through
the keytable entries to address this.

Signed-off-by: Justin Stephenson <jstephen@redhat.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>
Autobuild-User(master): Alexander Bokovoy <ab@samba.org>
Autobuild-Date(master): Sat Nov 10 12:48:02 CET 2018 on sn-devel-144