tests/krb5: Fix checksum generation and verification

tests/krb5: Fix checksum generation and verification

The KDC and server checksums may be generated using the same key, but
only the KDC checksum should have an RODCIdentifier. To fix this,
instead of overriding the existing methods, add additional ones for
RODC-specific signatures, so that both types of signatures can be
generated or verified.

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>