Stabilise and remove insignificant marker from tst_qquickflickable.

A lot of work has gone into this test already since
a79839dc5287de1448d1fd858db312a7bfa7b581, such as
6ba7d88df623df5bce85bfdae853fc49006e76f2.

Everything passes for me locally, so let's skip specific tests if they break (or
better: fix them) instead of having no test coverage at all.

Change-Id: I29a7aeb26b106955c442da26df486cb6ce4bc92a
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>