Improve GEE test coverage

[pjcozzi]

These files have some of the lowest coverage in Cesium:

Perhaps it is possible to create stubs to cover more of the code.

[pjcozzi]

And these:

[mramato]

Part of the issue is that there is a large amount of generated code from the protocol buffer stuff, it would make sense to add whatever special comments we need to instruct coverage to ignore those junks of code.

[pjcozzi]

Sounds good. Related: if it is not a huge amount of work, I am also into looking at other coverage tools that we can run as part of CI.

[mramato]

Switching to istanbul has been the plan for a while and is what we use in all of our other projects.. The problem is karma is lagging behind istanbul versions and we need the newer versions to actually be able to test Cesium (because of it's size). It might be easy to just use istanbul directly, but I need to look more into it.

[ggetz]

This is unlikely to become a priority anytime soon. We'd still happily accept a PR addressing this issue. But tracking this isn't particularly useful because of its low priority, so I will close it. Thanks!