selftests: tc-testing: use parallel tdc in kselftests

Leverage parallel tests in kselftests using all the available cpus.
We tested this in tuxsuite and locally extensively and it seems it's ready for prime time.

Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>