selftests/tc-testing: Fix warning and style check on tdc.sh

Replace exit code check with '! cmd' and add both quote and $(...)
around 'nproc' to prevent warning and issue reported by shellcheck.

Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
Link: https://patch.msgid.link/20250722095915.24485-5-chia-yu.chang@nokia-bell-labs.com


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