selftests: tc-testing: cleanup on Ctrl-C

Cleanup net namespaces and other resources if we get a SIGINT (Ctrl-C).
As user visible resources are allocated on a per test basis, it's only
required to catch this condition when (possibly) running tests.

So far calling post_suite is enough to free up anything that might
linger.

A missing keyword replacement for nsPlugin is also included.

Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://lore.kernel.org/r/20231124154248.315470-5-pctammela@mojatatu.com


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