selftests: netfilter: fix transaction test script timeout handling

The kselftest framework uses a default timeout of 45 seconds for
all test scripts.

Increase the timeout to two minutes for the netfilter tests, this
should hopefully be enough,

Make sure that, should the script be canceled, the net namespace and
the spawned ping instances are removed.

Fixes: 25d8bced ("selftests: add script to stress-test nft packet path vs. control plane")
Reported-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Signed-off-by: Florian Westphal <fw@strlen.de>
Tested-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>