Major test refactoring

This is a major refactoring to improve testability and support for the new download builder. Affecting issues #270 and #289, also mentioned in #369.

- Tests run standalone without any additional setup (just clone the CanJS repository), no separate CI setup necessary
- Plugin tests now run with all libraries
- Distributable files are being tested properly
- All tests are Steal independent
- Adds a `builder.json` configuration for both, the new download builder and the tests