chore(ci): Log tcp traffic on CircleCI (#3971)

A little bit excessive, but this will really help debugging issues with
Blocking Proxy.

Author: heathkit

circle.yml

@@ -19,6 +19,8 @@ dependencies:
     - cd testapp && npm update
     - npm start:
           background: true
+    - sudo tcpdump -i lo 'tcp && dst localhost' -w $CIRCLE_ARTIFACTS/localdump.pcap:
+          background: true
 
 test:
   override: