fix(build): ember-auto-import@2 compatibility

[buschtoens]

Makes this addon compatible with ember-auto-import@2 host apps.

Closes #25.

This is not the ideal solution, but the most trivial one.

As this addon is considered dormant and its features are set to be upstreamed into ember-qunit, I don't expect the owner @workmanw to cut a new release. Instead affected users may use patch-package to apply this fix locally.

[ef4]

The other thing I'd recommend is dropping this auto-registration entirely and telling users to do this import themselves, from their test-setup.js file.