s4:selftest: test kinit with the interdomain trust user account

s4:selftest: test kinit with the interdomain trust user account

To test it, add a blackbox test that ensures we pass a keytab-based
authentication with the trust user account for a trusted domain.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13539

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(cherry picked from commit 7df505298f71432d5adbcffccde8f97c117a57a6)