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

tools/testing/selftests/tc-testing/plugin-lib/nsPlugin.py

@@ -74,8 +74,7 @@ class SubPlugin(TdcPlugin):
             print('{}.post_suite'.format(self.sub_class))
 
         # Make sure we don't leak resources
-        for f in os.listdir('/run/netns/'):
-            cmd = self._replace_keywords("$IP netns del {}".format(f))
+        cmd = "$IP -a netns del"
 
         if self.args.verbose > 3:
             print('_exec_cmd:  command "{}"'.format(cmd))