selftests: forwarding: tc_actions: cleanup temporary files when test is aborted

remove temporary files created by 'mirred_egress_to_ingress_tcp' test
in the cleanup() handler. Also, change variable names to avoid clashing
with globals from lib.sh.

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Link: https://lore.kernel.org/r/091649045a017fc00095ecbb75884e5681f7025f.1676368027.git.dcaratti@redhat.com


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