selftests/tc-testing: cls_fw: add tests for classid

As discussed in '76e42ae8', cls_fw was handling the use of classid
incorrectly. Add a few tests to check if it's conforming to the correct
behaviour.

Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>