Review and improve Testing documentation

Currently the testing docs (https://github.com/adopted-ember-addons/ember-keyboard/blob/master/tests/dummy/app/templates/testing.hbs) describe using a different set of helper methods for integration tests vs. acceptance tests. This may not be true any longer since Ember has unified these APIs. Someone needs to do some tests and research and update this page.

Read tests/dummy/app/templates/testing.hbs and compare its integration-test and acceptance-test helper guidance with the current Ember APIs. Use the dummy app to test whether the APIs are unified as described, then update the page so its instructions match the verified behavior.

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