Improve testing of Socket Tagging code

Support for Android socket tagging is added in PR #2423. However, the test coverage is not ideal. [Chromium's unit tests](https://source.chromium.org/chromium/chromium/src/+/main:net/socket/socket_tag_unittest.cc;l=81) for the socket tagging implementation looks at `/proc` to [check that traffic was actually tagged](https://source.chromium.org/chromium/chromium/src/+/main:net/socket/socket_test_util.cc;drc=8f20fb0fd2ed4ccb7dd703557c34c3afd798f49b;l=2122). 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.