selftests: tc-testing: use netns delete from pyroute2

When pyroute2 is available, use the native netns delete routine instead
of calling iproute2 to do it. As forks are expensive with some kernel
configs, minimize its usage to avoid kselftests timeouts.

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/20231117171208.2066136-4-pctammela@mojatatu.com


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