chore(test): rename RootTestComponent to ComponentFixture

BREAKING CHANGE:

Before:

```
testComponentBuilder.createAsync(MyComponent).then(root: RootTestComponent => {
}
```

After:

```
testComponentBuilder.createAsync(MyComponent).then(fixture: ComponentFixture => {
}
```

Closes #4711

modules/angular2/testing.ts

@@ -7,6 +7,6 @@
  *
  */
 export * from './src/testing/testing';
-export {RootTestComponent, TestComponentBuilder} from './src/testing/test_component_builder';
+export {ComponentFixture, TestComponentBuilder} from './src/testing/test_component_builder';
 export * from './src/testing/test_injector';
 export * from './src/testing/fake_async';