Test coverage for Content Security Policy (CSP)

We lost the test coverage that the addon is compatible with a strict content security policy (CSP) in #218. We should add it back.

It was previously implemented using [ember-cli-content-security-policy](https://github.com/rwjblue/ember-cli-content-security-policy). Assuming that it worked before, adding that dependency back to the test-app might be all what is needed.