Exclude failing table tests from Travis

As recently reported by @dominikl, @jburel and others, Travis intermittently fails with 2 of the table tests with an `Internal Exception`. This intermittent failure is captured by https://trac.openmicroscopy.org.uk/ome/ticket/12372. 

Until someone takes the time to investigate what goes wrong there, this PR marks these tests as `xfail` so that these intermittent failures do not affect our workflow as they do now.