selftests: tc-testing: report number of workers in use

Report the number of workers in use to process the test batches.
Since the number is now subject to a limit, avoid users getting
confused.

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/20231117171208.2066136-7-pctammela@mojatatu.com


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