selftests: concurrency: add test to verify parallel rules insertion

Implement test that verifies parallel rules insertion by adding 1 million
flower filters with 10 concurrent tc instances. Put it to standalone
'concurrency' category.

Implement tdc_multibatch.py helper script that is used to generate multiple
batch files for concurrent tc execution. Extend config with new 'BATCH_DIR'
variable to specify temporary output directory that is used to store batch
files generated by tdc_multibatch.py.

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