tests: Use MUST_USE_KERBEROS over AUTO_USE_KERBEROS in auth_log tests

tests: Use MUST_USE_KERBEROS over AUTO_USE_KERBEROS in auth_log tests

The s3 SMB client bindings seem slightly different to s4, in that they
default to setting the CLI_FULL_CONNECTION_FALLBACK_AFTER_KERBEROS flag.
This seems to fallback to finding a valid KRB TGT (from a previous
successful test), which results in the connection succeeding rather than
failing.

Setting MUST_USE_KERBEROS explicitly avoids this behaviour.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Jeremy Allison <jra@samba.org>