ember-testing: multiple app compatibility

## foo-app-test.js
// give each tested app its own DOM element
document.write('<div class="ember-testing-container"><div id="foo-app-testing"></div></div>');
FooApp.rootElement = '#foo-app-testing';
// defer readiness
FooApp.setupForTesting();

module("Foo App", {
  setup: function() {
    FooApp.injectTestHelpers();
    // last step, before each test begins, clears the container and advances readiness
    FooApp.reset();
  },
  teardown: function() {
    // clear the helpers everytime so that a different app can inject them
    FooApp.removeTestHelpers();
  }
});