message: test: fix race condition in FakeUpstream.
  Dispatcher and client threads were racing on connection callback registration, leaving the callback
  list in a bad state. This fixes the hangs we see from time to time in the integration tests.
  Validated with bazel test //test/integration/... --runs_per_test=1000.
diff: @@ -74,7 +74,7 @@ public:
  protected:
  FakeConnectionBase(Network::Connection& connection) : connection_(connection) {
  - connection.addConnectionCallbacks(*this);
  + connection_.dispatcher().post([this]() -> void { connection_.addConnectionCallbacks(*this); });
  }
  Network::Connection& connection_;
  