BUG: Fix qMRMLSliceControllerWidgetTest

Following r22568, the controller has been updated so that the abbreviated
slice name can be dynamically updated after the "LayoutLabel" has been
changed on the associated slice node.

This commit clean up the code removing un-needed ivar "SliceViewLabel" and
fix the test.

Finally, also set "sliceViewLabel" as a property so that is accessible
from python.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22572 3bd1e089-480b-0410-8dfb-8563597acbee
STYLE: Remove unused "SliceOrientation" ivar from SliceControllerWidget

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22573 3bd1e089-480b-0410-8dfb-8563597acbee