selftests: concurrency: add test to verify concurrent delete

Implement test that verifies concurrent deletion of rules by executing 10
tc instances that delete flower filters in same handle range. In this case
only one tc instance succeeds in deleting a filter with particular handle.
To mitigate expected failures of all other instances, run tc with 'force'
option to continue processing batch file in case of errors and expect xargs
to return code '123' that indicates that invocation of command(s) exited
with error in range 1-125.

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