Fix wrong number of channels for images in fake SPW (fix #12501)

This PR fixes https://trac.openmicroscopy.org.uk/ome/ticket/12501.

To test:
- create a fake SPW dir/file structure with ``bioformats/tools/mkfake`` (see ticket for specific command)
- import to OMERO
- verify that well images are displayed correctly

/cc @melissalinkert, @dpwrussell, @jburel
This PR changes the default channel count for images generated in ``XMLMockObjects``. I think there shouldn't be any negative results of this change.