tst_qquickimage seems flaky

tst_qquickimage was failing only on Linux 32/64-bit in a recent CI run. I could reproduce the failure locally; it seemed to be caused by missing calls to QTest::qWaitForWindowShown(). There are a lot of places in that autotest that call QQuickView::show() without calling qWaitForWindowShown(). I also get the warning "QOpenGLContext::swapBuffers() called with non-exposed window, behavior is undefined" for some of the test functions.

Tested marked as insignificant in commit 3e6a8eca00334df344a45f09afbcf8fd8e2b7c54.