Doc: explain how to ensure the TestCase's window is active

Pick-to: 6.5 6.6 6.7
Change-Id: I0d2e8a8ffdabba9cd6f0aee7dad8e79728c0261b
Reviewed-by: Oliver Eftevaag <oliver.eftevaag@qt.io>

src/qmltest/TestCase.qml

@@ -166,6 +166,15 @@ import "testlogger.js" as TestLogger
     and mouseMove() methods can be used to simulate mouse events in a
     similar fashion.
 
+    If your test creates other windows, it's possible that those windows
+    become active, stealing the focus from the TestCase's window. To ensure
+    that the TestCase's window is active, use the following code:
+
+    \code
+    testCase.Window.window.requestActivate()
+    tryCompare(testCase.Window.window, "active", true)
+    \endcode
+
     \b{Note:} keyboard and mouse events can only be delivered once the
     main window has been shown.  Attempts to deliver events before then
     will fail.  Use the \l when and windowShown properties to track