docs(core): move core examples into `examples/core/` directory

This allows examples to be found during aio's `yarn serve-and-sync`, which only
looks for examples in `packages/examples/<packageName>/**/*`, where
`packageName` is the name of the package that the modified file belonged to;
e.g. `core`, `common`, etc.).

packages/core/testing/src/fake_async.ts

@@ -40,7 +40,7 @@ let _inFakeAsyncCall = false;
  *
  * ## Example
  *
- * {@example testing/ts/fake_async.ts region='basic'}
+ * {@example core/testing/ts/fake_async.ts region='basic'}
  *
  * @param fn
  * @returns The function wrapped to be executed in the fakeAsync zone
@@ -107,7 +107,7 @@ function _getFakeAsyncZoneSpec(): any {
  *
  * ## Example
  *
- * {@example testing/ts/fake_async.ts region='basic'}
+ * {@example core/testing/ts/fake_async.ts region='basic'}
  *
  * @experimental
  */