tests/krb5: Improve edata checking

tests/krb5: Improve edata checking

Instead of guessing based on a heuristic whether we have KERB_ERROR_DATA
or METHOD_DATA in the ‘e-data’ field, decode it first as KERB_ERROR_DATA
and fall back to METHOD_DATA if that fails.

The environment variable EXPECT_NT_STATUS indicates that the KDC
supports returning a status code in the e-data field.

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