Add integration tests

[nkreeger]

It would be great to have:

• Automatic tests from WebGL conformance tests
• Jasmine tests for specific functionality

[AlexVestin]

I managed to run the conformance tests using nw.js, not sure if this is the best way to do it though

[nkreeger]

Most of the unit testing we have is indirectly through an upcoming backend to tensorflow.js:
https://github.com/tensorflow/tfjs-core/tree/master/tfjs-backend-nodegl

Run test:
https://github.com/tensorflow/tfjs-core/blob/master/tfjs-backend-nodegl/package.json#L10

Test file:
https://github.com/tensorflow/tfjs-core/blob/master/tfjs-backend-nodegl/src/run_tests.ts

That runs the whole stack against the existing browser WebGL unit tests for TensorFlow.js