• Add a dead-simple way to stub/mock Window.fetch when using describes.

Related: #10128 (comment) and #9338

Internal discussion: https://groups.google.com/a/google.com/d/msg/amphtml-eng/YKxMVlpQ6xA/fj3KS3FDAQAJ

/cc @lannka @taymonbeal @jridgewell @rsimha-amp