selftests: tc-testing: leverage -all in suite ns teardown

Instead of listing lingering ns pinned files and delete them one by one, leverage '-all'
from iproute2 to do it in a single process fork.

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/20231117171208.2066136-5-pctammela@mojatatu.com


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