jest: Use jest-expo as our preset.

This calls `react-native`'s Jest setup code before running its own.
In particular, it seems to be aware of all the Expo modules we might
want to add using `react-native-unimodules`, and mocks them [1].
Since we don't need our own mocks for these, remove them.

It seems like we still need to add entries in our
`transformModulesWhitelist`, ah, well. But it's good to weed out
boring mocks from our `jestSetup.js`, and leave only those that are
interesting [2]. Also, it seems like each time we add a module from
Expo, there's a debugging process that can be confusing [3]; so,
nice to avoid that.

It looks like the preset does explicitly consider the bare,
non-"managed" Expo workflow [4], which we use.

If `jest-expo` turns out to be buggy, or the dependency requirements
get even more tangled or burdensome, we should feel free to abandon
this effort; it's not terrible to have to add boring mocks.

[1]: https://github.com/expo/expo/blob/b8bd30697/packages/jest-expo/src/preset/expoModules.js
[2]: Seems like a few remain that aren't related to Expo. Hmm.
[3]: https://github.com/zulip/zulip-mobile/pull/4034/files#r445956933
[4]: https://github.com/expo/expo/blob/b8bd30697/packages/jest-expo/src/preset/setup.js#L130

jest.config.js

@@ -4,6 +4,7 @@
 // These will be used as regexp fragments.
 const transformModulesWhitelist = [
   'expo-apple-authentication',
+  'expo-application',
   'react-native',
   // @rnc/async-storage itself is precompiled, but its mock-helper is not
   '@react-native-community/async-storage',
@@ -24,7 +25,7 @@ const transformModulesWhitelist = [
 const transformIgnorePattern = `node_modules/(?!${transformModulesWhitelist.join('|')})`;
 
 module.exports = {
-  preset: 'react-native',
+  preset: 'jest-expo',
 
   // Finding and transforming source code.
 

jest/jestSetup.js

@@ -54,18 +54,7 @@ jest.mock('react-native-simple-toast', () => ({
   showWithGravity: jest.fn(),
 }));
 
-jest.mock('expo-application', () => ({
-  nativeApplicationVersion: '26.23.146',
-}));
-
 jest.mock('react-native-device-info', () => ({
   getSystemName: jest.fn().mockReturnValue('ios'),
   getSystemVersion: jest.fn().mockReturnValue('13.3.1'),
 }));
-
-jest.mock('expo-apple-authentication', () => ({
-  AppleAuthenticationButton: jest.fn(),
-  isAvailableAsync: jest.fn(),
-  signInAsync: jest.fn(),
-  // etc. (incomplete)
-}));