tests/krb5: Get encpart decryption key from kdc_exchange_dict

tests/krb5: Get encpart decryption key from kdc_exchange_dict

Instead of using check_padata_fn to get the encpart decryption key, we
can get the key from the AS-REQ preauth phase or from the TGT, depending
on whether the message is an AS-REQ or a TGS-REQ. This allows removal of
check_padata_fn and some duplicated code.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14642

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