WIP: Add integration tests for existing net_in/net_out functionality

 - Add 'netdog' executable to test udp
 - still some test pollution if run alongside lifecycle tests
 - tests for deny_networks fail unexpected for containers in the same
 subnet unless old/linux_backend/skeleton/net.sh#L42 is commented out
 - ideally use configurable external IP to talk to instead of relying on
 8.8.8.8 existing

[#81502222]