Spying on CesiumMath is broken in tests

Looks like the built tests have their own version of CesiumMath, this is most likely related to the global replacement we do for the Cesium instance. The end result is that something like:

```js
spyOn(CesiumMath, 'nextRandomNumber').and.returnValue(1234);
```

Will turn into a silent no-op and not affect the CesiumJS code actually calling Cesium.Math

Start by reproducing the CesiumMath spy example in the built tests, then trace how the global Cesium replacement provides CesiumMath to the test and to the CesiumJS code under test. Done means spying on CesiumMath.nextRandomNumber changes the value observed by the actual Cesium.Math caller.

Written by the indexing model from the issue text.