Adding note to testing.md about qunit-dom

[tsengia]

This PR just adds a small snippet to the testing.md page about importing qunit-dom so that Typescript is aware of the assert.dom() function commonly used in Component tests.

It took me a while to figure this out, so I hope that this small snippet can save others some time in the future.

[chriskrycho]

Ah, this is good to have here. qunit-dom's own docs also cover this, and I believe the blueprints shipped in ember-source do the right thing, but this is great to have. Thanks!