tests/krb5: Request only supported encryption types in get_tgt()

tests/krb5: Request only supported encryption types in get_tgt()

If the domain uses functional level 2003, calling get_tgt() would
request an AES256-encrypted ticket. The KDC would respond to that
request with incorrect etype-info, and were it not for many tests lying
(via client_as_etypes) about what etypes were supported, those tests
would fail pointlessly.

As this behaviour is not what get_tgt() is intended to test, we now only
request etypes that are actually supported.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>