Summary:
`Fake` components are simplified so snapshots are stable and reliable, and references are exported
so that interactions like `onRefresh` and `onScroll` can be called manually. Currently there is just
one global export for each class, but we may change this in the future if we need to manage multiple
`Fake`s of the same type in one render tree.
Right now these must be installed explicitly, but I might move them into `__mocks__` folders if it
seems reasonable to make them defaults.
Reviewed By: cpojer
Differential Revision: D4318207
fbshipit-source-id: 62802353a98b09ca1c80804ef7201ea63091f94a
Spencer Ahrens