Rename to @ember/test-helpers

[Turbo87]

With the introduction of RFC 176 Ember moved towards import paths prefixed with @ember/ which ultimately should use the @ember scope on npm. This gives us a better indicator of a project being official or a community-project. Since ember-test-helpers is an official project it might make sense to move it to @ember/test-helpers too.

emberjs/rfcs#268 is introducing a public API for this package for the first time so we should probably decide this until the RFC lands.

/cc @rwjblue

[rwjblue]

👍 I am in favor of this. Since this package is generally provided as a transitive dependency, the only real issue would be to ensure that the current ember-test-helpers imports continue to work after the transition (which should be pretty easy).