BUG: Fix logic checking for readonly folder in writeToMRB method

Following r22395 "qSlicerSceneWriter::writeToMRB" has been updated to
check is the MRB file to be written was writable or not, the test was
always failing since the file itself wasn't existing.

This commit implement the appropriate logic by checking if the destination
folder where the MRB should be saved is writable.

This commit also fix the test "test_slicer_util_save.test_saveSceneAsMRB"

See #3242

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22401 3bd1e089-480b-0410-8dfb-8563597acbee