[Thread] introduce TestTunNetworkUtils and use it in existing test cases

TestTunNetworkUtils is a centralized place to control the test TUN networks used by integration test. With this we can safely tear down all test networks in `teardown()` instead of tearing down the newly created network in `finally` clause.

Bug: 377640059
Change-Id: I57b34bb2644bd6a5bb73ed1573932f35de657c8f
4 files changed