Fix ColumnLayout unit tests

Short version: `fillVerticalSpace` is `false` by default. Setting it to `true` fixes both tests.

I did some more refactoring of the test class to make things a bit clearer.

Tested on Jenkins
http://jenkins.terasology.org/job/TerasologyTemp/lastCompletedBuild/testReport/

This fixes the remaining broken unit tests. As a result the Jenkins build is now **stable** again.

Fixes #1393