test: improve test-http-agent-destroyed-socket.js

* wrap callbacks in mustCall()
* Wrap the callbacks which make assertions in common.mustcall()
  to ensure they are called

PR-URL: https://github.com/nodejs/node/pull/11201


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>