Fix libustctl_function_tests

After discussions, we concluded that the enable a non existing marker is a valid case, so we
move it to the working case section.
While being there, check that the re-enable a marker set the right errno

Signed-off-by: Yannick Brosseau <yannick.brosseau@gmail.com>
Signed-off-by: Nils Carlson <nils.carlson@ericsson.com>