Add more JS/IDL tests

[greggman]

I recently ran into an issue that some properties of GPUAdapter and GPUDevice where not enumerable in dawn.node. I fixed that but, a quick test finds other issues.

One example:

adapter = navigator.gpu.requestAdapter();
adapter.featureLevel = 123;         // works in browser, throws in dawn.node ("only getter")
console.log(adapter.featureLevel);  // prints the feature level (not 123) in the browser (as expected)

It would be good to know all these properties behave the same across implementations.

[greggman]

related? #4139