selftests: tc-testing: cap parallel tdc to 4 cores

We have observed a lot of lock contention and test instability when running with >8 cores.
Enough to actually make the tests run slower than with fewer cores.

Cap the maximum cores of parallel tdc to 4 which showed in testing to
be a reasonable number for efficiency and stability in different kernel
config scenarios.

Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://lore.kernel.org/r/20231117171208.2066136-2-pctammela@mojatatu.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>