Improve tst_QMediaPlayerBackend::processEOS()

When at the end of the media, check that pause() resets the position to
the beginning and that the status changes to BufferedMedia.

Change-Id: I3c7aeb98284de8cbeb12195fbf0a308e593af67b
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
Reviewed-by: Christian Stromme <christian.stromme@qt.io>