pixi/core tests

Wrote some tests to see coverage go up (at least on local reports ATM).

Notes:
- I'm not sure if I should test any of the `mat4` and `mat3.toMat4` stuff, as it is not used anywhere in pixi
- I'm not sure of the proper way to test the `mat3` methods
- There are some problems with the `contains` methods in `PIXI.Ellipse` and `PIXI.Rectangle`
- I didn't test the `PIXI.Polygon` `contains` method