Unit tests have gotten slow

Cesium's unit tests used to run in 45 seconds in Chrome, over the past year they have crept up to 3 minutes! Older slow browsers (IE) can take as long as 10 minutes.

I'm fairly certain that there are only one or two severely long running tests that are causing the problem. We should identify them and also put checks in place to ensure that they are not re-introduced.

Start by running Cesium's unit-test suite in Chrome and an older browser, then identify the one or two tests responsible for the increase from 45 seconds to several minutes. Done means the slow tests are identified and checks are in place to prevent their runtime from regressing.

Written by the indexing model from the issue text.