Isolate PingStressTest from the internet

Remove dependence on internet services; what we really want to test is
being able to send a routable packet to the router. Everything after the
router is not the DUT's responsibility.

Perform some cleanup while touching the iperf_server and utils files.

Bug: 232148496
Bug: 232148609
Fix: 232148496
Test: act.py [...] -tc PingStressTest
Change-Id: I720e63d7d39ad7a94e60a7440a81c20b3d7b6546