selftests: implement flower classifier terse dump tests

Implement two basic tests to verify terse dump functionality of flower
classifier:

- Test that verifies that terse dump works.

- Test that verifies that terse dump doesn't print filter key.

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>