Ensure any top level `tests/` folder is included in testPackage

[rondale-sc]

This resolves https://github.com/glimmerjs/glimmer-application-pipeline/pull/128

However, the Glimmer application blueprint needs to be updated to do a variety of thigns.

• [ ] Must update testem.js to point it's test_page directive to dist/tests/index.html
• [ ] Must update the blueprint dependency on this package
• [ ] Must create a default tests/index.html file that conforms to the latest QUnit CDN url (temporarily) and requires the test entry point.