CVE-2020-25719 CVE-2020-25717 tests/krb5: Modify get_service_ticket() to use _generic_kdc_exchange()

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14799
BUG: https://bugzilla.samba.org/show_bug.cgi?id=14561

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