Prevent NetworkTraceHandlerTest from touching BPF.

Despite extending the handler with a test-only version, calling
HandlerForTest::Register creates a NetworkTraceHandler instead. This
change adds a class-level toggle to skip certain code in tests. Without
this, it's possible for the test to see system-level packets and fail
(although I haven't seen this yet).

Bug: 246985031
Test: atest libnetworkstats_test w/ logs in the poller Start/Stop
Change-Id: If22d91e5449bc774961c58115ba84ca2a4bcde59