[1/3] Support isolated containers for parallel packetimpact tests

Summary of the approach: the test runner will set up a few DUTs according to
a flag and pass all the test networks to the testbench. The testbench will only
reside in a single container. The testbench will put all the test networks into
a buffered channel which served as a semaphore and now the user can freely use
t.Parallel() in (sub)tests and the true parallelism will be determined by how
many DUTs are configured. Creating DUTs on demand is not supported yet, the
test author should determine the number of DUTs to be used statically.

Specifically in this change:
- Don't export any global variables about the test network in testbench.
- Sniffer only binds on the local interface because it will be possible to have
  multiple interfaces to multiple DUTs in a single testbench container.
- Migrate existing tests to stop using global variables.

PiperOrigin-RevId: 343965962

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