tst_QHostInfo: increase the timeout to 15s

That seems to be enough for me when trying to reverse-resolve host names
inside the corporate network.

Change-Id: I6818d78a57394e37857bfffd17bc6b0ebd39f51b
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit d18d8a7ad1f69b2e65de114d9a3edc23d56da30e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/network/kernel/qhostinfo/tst_qhostinfo.cpp

@@ -127,7 +127,7 @@ public:
         });
     }
 
-    bool waitForResults(std::chrono::milliseconds timeout = 10s)
+    bool waitForResults(std::chrono::milliseconds timeout = 15s)
     {
         QTestEventLoop::instance().enterLoop(timeout);
         return !QTestEventLoop::instance().timeout() && lookupDone;