Warn if mouse event occurs outside target window in tests.

This class of bug seems to occur in 'flaky' CI tests, so give
more information about what the problem is. Found here:

 https://codereview.qt-project.org/#change,44210

Change-Id: I73908e309bdecf1fd3bc323dd4e8febe47c1239a
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>

src/testlib/qtestmouse.h

@@ -84,6 +84,9 @@ namespace QTest
         QTEST_ASSERT(window);
         extern int Q_TESTLIB_EXPORT defaultMouseDelay();
 
+        if (!window->geometry().contains(pos))
+            QTest::qWarn("Mouse event occurs outside of target window.");
+
          static Qt::MouseButton lastButton = Qt::NoButton;
 
         if (delay == -1 || delay < defaultMouseDelay())