Clean up unit test log spam

[joshlory]

Trap these messages so they don't end up in TravisCI output:

Phaser.Cache.getImage: Key "blocks" not found in Cache.
Cannot set frameName: Grass
Phaser.Cache.getImage: Key "blocks" not found in Cache.
Cannot set frameName: Grass
Phaser.Cache.getImage: Key "blocks" not found in Cache.
Cannot set frameName: Grass
...

Sample build: https://travis-ci.org/code-dot-org/craft/builds/287173536