Avoid race condition in QQuickTests

Avoid race condition in QQuickTests

The QQuickView::show() function may or may not be synchronous and
because the rendering happens on another thread, the frameSwapped
can be emitted before we enter the qWaitForSignal() event loop.
Fix this by waiting for the window to become exposed instead, which
implies that it has rendered at least one frame.

Change-Id: I1100a6fe981018395bf141d67f06adb94a354206
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>