Fix wrong number of channels for images in fake SPW (fix #12501) (rebased onto dev_5_0)


This is the same as gh-1242 but rebased onto dev_5_0.

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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.