tests/dcerpc/raw_protocol: add more test for auth padding during ALTER_CONTEXT/AUTH3

tests/dcerpc/raw_protocol: add more test for auth padding during ALTER_CONTEXT/AUTH3

The aim is to keep testing the code paths, which are no longer
testing because allow_bind_auth_pad is false now, which
means the existing tests fail directly at the BIND,
but we also want to test the error handling on
ALTER_CONTEXT (and AUTH3).

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14356

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>