selftests: tc-testing: add one test for flushing explicitly created chain

selftests: tc-testing: add one test for flushing explicitly created chain

Add the test for additional reference to chains that are explicitly
created by RTM_NEWCHAIN message.

The test result:

 1..1
 ok 1 c2b4 - soft lockup alarm will be not generated after delete the prio 0
  filter of the chain

This is a follow up to commit c9a82bec02c3 ("net/sched: cls_api: Fix lockup on flushing explicitly created chain").

Signed-off-by: Mingshuai Ren <renmingshuai@huawei.com>
Acked-by: Pedro Tammela <pctammela@mojatatu.com>
Acked-by: Victor Nogueira <victor@mojatatu.com>
Link: https://lore.kernel.org/r/20230620014939.2034054-1-renmingshuai@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>