There should be at least a test with a material different from 'PbrMaterial'

[setzer22]

To ensure custom workflows are covered by the test suite to some extent, the tests should be using something other than PbrMaterial. This way issues like #371 can be caught earlier on.

Something like a test scene creating a custom material type with a simple shader, then adding a few objects using that material.

[cwfitzgerald]

An example would be good also to show off what you have to do as a user, we should also split out unlit materials from the pbr material as that way they can get a much lighter shader.