Add opt-in integration with @ember/test-waiters

[SergeAstapov]

There are use cases when you may want css-transitions to integrate with test helpers provided by @ember/test-helpers package. This is not possible today out of the box.

This PR allows users to specify useTestWaiters config option in ember-cli-build.js file like so:

  const app = new EmberApp(defaults, {
    '@embroider/macros': {
      setConfig: {
        'ember-css-transitions': {
          useTestWaiters: true
        }
      }
    },
    // other options
  });