Restore ability to run specs against built versions of cesium.

[mramato]

Since the specs use ES6 modules and built Cesium is an ES5 UMD, I added a build-specs build step which generates an combined ES5 version of the specs which rely on Cesium as a global variable. We then inject these files into jasmine instead of the standard specs and everything works exactly as it did before.

It was deceptively simple once I had my eureka moment.

[cesium-concierge]

Thanks for the pull request @mramato!

• ✔️ Signed CLA found.
• ❔ Changes to third party files were made.
  • Looks like a file in one of our ThirdParty folders (ThirdParty/, Source/ThirdParty/) has been added or modified. Please verify that it has a section in LICENSE.md and that its license information is up to date with this new version.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.