rewrote InterpolationTest

- added time marker in the graph
- added ability to change duration using ctrl + scroll
- path is now drawn
- doesn't need any assets
I know this isn't of any importance, but I had to rewrite it anyway, so I added a few things.
Update InterpolationTest.java
replaced generated assets with test assets
fixed stupid NPE

Skin was local
Merge pull request #861 from dermetfan/master

rewrote InterpolationTest