jest: Mock react-native ourselves.

The immediate need is to set up a way to mock the new `ZLPConstants`
in `NativeModules`, which we'll do in an upcoming commit.

Doing this, as recommended by React Native [1], also means we're
better prepared for the React Native v0.61 upgrade (#3781), in which
Haste is removed [2]. A consequence of that removal, it seems, is
that mocks like this one, which we have now:

```
jest.mock('Linking', () => { ... }`
```

, won't work. Several people have handled this by changing 'Linking'
to something like 'react-native/Libraries/Linking/Linking', but this
is brittle because it couples our tests with the current directory
structure in 'react-native'. Better to do it this way.

We considered following the advice of others at that issue,
including a blog post [3] responding to the official suggestion with
an alternative. But we didn't reproduce the problems the post's
author mentioned, and we've so far been able to explain the hiccups
we've seen.

[1] facebook/react-native#26579 (comment)
[2] facebook/react-native#26579 (comment)
[3] facebook/react-native#26579 (comment)

jest/jestSetup.js

@@ -1,4 +1,46 @@
 import mockAsyncStorage from '@react-native-community/async-storage/jest/async-storage-mock';
+import * as ReactNative from 'react-native';
+
+// Following upstream advice [1], mock `react-native` ourselves.
+//
+// Helpful in particular for adding to NativeModules, to simulate
+// modules being available after the bridge initializes.
+//
+// The 'react-native' we import here is already mocked to a great
+// extent by `node_modules/react-native-jest/setup.js`, thanks to our
+// having declared `preset: 'react-native'` in our Jest config.
+//
+// For how we fall back on these already-mocked properties:
+//
+// Following the suggested implementation, we don't do something like
+// an object spread, but instead set the returned object's prototype
+// to be ReactNative. They don't explain why, but at least one reason
+// seems to be to avoid triggering deprecation warnings on accessing,
+// e.g., ReactNative.CameraRoll, which would would be done during a
+// spread as each property is enumerated.
+//
+// [1] https://github.com/facebook/react-native/issues/26579#issuecomment-535244001
+
+jest.mock('react-native', () =>
+  // Extend ReactNative
+  Object.setPrototypeOf(
+    {
+      // We don't mock anything in these two, but
+      // `react-native-vector-icons` uses an odd indirection: they
+      // have a `lib/react-native.js` file with just the following:
+      //
+      // `export * from 'react-native';`
+      //
+      // This seems to mean that the ReactNative prototype (which
+      // we're setting to be the prototype of the object created by
+      // this literal) is ignored. So, put these directly in the
+      // object literal.
+      Platform: ReactNative.Platform,
+      StyleSheet: ReactNative.StyleSheet,
+    },
+    ReactNative,
+  ),
+);
 
 jest.mock('@react-native-community/async-storage', () => mockAsyncStorage);