testlib: Explicitly name watchdog timer thread

Simplifies debugging of issues where the watchdog thread is involved.

Change-Id: I4862167bca4a942c7d4319a9374f1f83f292d831
Reviewed-by: Simon Hausmann <hausmann@gmail.com>

src/testlib/qtestcase.cpp

@@ -1026,6 +1026,7 @@ class WatchDog : public QThread
 public:
     WatchDog()
     {
+        setObjectName(QLatin1String("QtTest Watchdog"));
         auto locker = qt_unique_lock(mutex);
         expecting = ThreadStart;
         start();