Bug 991442 - Add coverage threshold to test-agent

https://bugzilla.mozilla.org/show_bug.cgi?id=991442
I think you forgot something here.

`this.paths` is the property that's set in constructor, and it's an array of paths. You need to iterate and fetch it using XHR and use JSON.parse. Please don't use "require" to load json, this is insecure :)

Thanks