Check that we can actually read from network and not just connect

When using Cloudflare, as we do for www.wxwidgets.org, the connection
succeeds as long as Cloudflare itself works, but reading later fails if
the real server behind Cloudflare proxy does not, so check if we can
read something from it.

tests/test.cpp

@@ -442,9 +442,13 @@ extern bool IsNetworkAvailable()
         return false;
     }
 
+    const char* const
+        HTTP_GET = "GET / HTTP /1.1\r\nHost: www.wxwidgets.org\r\n\r\n";
+
     wxSocketClient sock;
     sock.SetTimeout(10);    // 10 secs
-    bool online = sock.Connect(addr);
+    bool online = sock.Connect(addr) &&
+                    (sock.Write(HTTP_GET, strlen(HTTP_GET)), sock.WaitForRead(1));
 
     wxSocketBase::Shutdown();