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

As discussed in 'b80b829e

', cls_route was handling the use of classid
incorrectly. Add a test 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>