tests/krb5: Use consistent ordering for etypes

tests/krb5: Use consistent ordering for etypes

The 'etype' field in a Kerberos request is ordered. Make this fact
clearer by using a tuple or an array to represent etypes rather than a
set.

get_default_enctypes() now returns encryption types in order of
strength. As a consequence, the encryption type chosen by the MIT KDC
matches up with that chosen by Windows, and more tests begin to pass.

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