Disable MTPixelDataTest for now

gh-3023 enabled unit tests for several OMERO components. Initial runs
of MTPixelDataTest passed on travis and on 2 different workstations.
Now, the NPE which was seen on one work station is "infecting" travis
as well. Disabling the test for now.
Merge pull request #3030 from joshmoore/disable-mt-pixel-test

Disable MTPixelDataTest for now