Use process.env.EMBER_ENV for env check.
Use process.env.EMBER_ENV for env check.

process.env.EMBER_ENV is always set (before evaluating addon code), whereas `this.app` is _only_ set for "normal apps".

Specifically, this fixes using `ember-a11y-testing` in an addon's dummy app.
ember-a11y/ember-a11y-testing,ember-a11y/ember-a11y-testing,ember-a11y/ember-a11y-testing