Fix tests that rely on a network connection

The specs excluded in [this commit](https://github.com/CesiumGS/cesium/commit/b85d8a449284c9d6f6bcc9fc4226e47cc0594630) on the `cesium.com` branch (which changes the Ion default access token) should be fixed to not have to rely on connection to Cesium ion. Test data should be local to the tests can run offline.

Start by inspecting the specs excluded in commit b85d8a449284c9d6f6bcc9fc4226e47cc0594630 on the cesium.com branch, then trace how their test data reaches Cesium ion. Make the test data local and run the affected specs offline; done means the tests no longer require a connection to Cesium ion.

Written by the indexing model from the issue text.