Fix test that guards forcible removal of dm partition nodes.

Since the superclass _postDestroy method sets self.exists to False, and
self.status can only be True for devices that exist, we cannot rely on
self.status to see if the device node remains.
devicePathToName should default to returning a basename.

This is specifically for the case of DiskFile, which has an
arbitrary file path, but it looks like the entire method could be
replaced with the basename call.
Pass a path (not a name) to devicePathToName.

Also, we were unfairly asking parted to make guesses about
getDeviceNodeName for arbitrary files outside of /dev.
Temporarily disable a test that isn't working.

Related: rhbz#1162823
Don't run selinux context tests when selinux is disabled.