Improve testing of Socket Tagging code

[RyanTheOptimist]

Support for Android socket tagging is added in PR #2423. However, the test coverage is not ideal. Chromium's unit tests for the socket tagging implementation looks at /proc to check that traffic was actually tagged. The current Envoy Mobile test environment runs with #ifdef(ANDROID_API) false so such tests aren't really possible.

Once we have support for running tests in an Android environment #2450, we should makes use of this capability to improve socket tagging tests.