selftests: tc-testing: Add testcases to test new flush behaviour

Add tdc test cases to verify new flush behaviour is correct, which do
the following:

- Try to flush only one action which is being referenced by a filter
- Try to flush three actions where the last one (index 3) is being
  referenced by a filter

Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>