Use @ember/test-waiters in @embroider/router

[mydea]

Currently, @embroider/router imports directly from @ember/test, which triggers the following runtime error for me when trying to run my app:

Uncaught Error: Attempted to use test utilities, but `ember-testing` was not included
    at testingNotAvailableMessage (webpack-internal:///./node_modules/ember-source/@ember/test/index.js:30)
    at Router.init (webpack-internal:///./node_modules/@embroider/router/index.js:119)
    at Router.superWrapper [as init] (webpack-internal:///./node_modules/ember-source/@ember/-internals/utils/index.js:445)
    at initialize (webpack-internal:///./node_modules/ember-source/@ember/-internals/runtime/lib/system/core_object.js:165)
    at Function.create (webpack-internal:///./node_modules/ember-source/@ember/-internals/runtime/lib/system/core_object.js:697)
    at FactoryManager.create (webpack-internal:///./node_modules/ember-source/@ember/-internals/container/index.js:535)
    at Proxy.create (webpack-internal:///./node_modules/ember-source/@ember/-internals/container/index.js:250)
    at instantiateFactory (webpack-internal:///./node_modules/ember-source/@ember/-internals/container/index.js:351)
    at lookup (webpack-internal:///./node_modules/ember-source/@ember/-internals/container/index.js:279)
    at Container.lookup (webpack-internal:///./node_modules/ember-source/@ember/-internals/container/index.js:146)

This PR adds @ember/test-waiters as a dependency to @embroider/router and uses it instead of registerWaiter.

[ef4]

Thanks, looks good!