jest: Use jest-expo/ios preset.

When we've used the `jest-expo` preset or the `react-native` preset,
the tests get run "in the standard React Native environment
(iOS)" [1]. The key piece of this is that they run with
`Platform.OS` mocked to 'ios'.

We're about to add another "project" to test Android codepaths with
`jest-expo/android`. First, though, swap out the old `jest-expo`
preset for this more explicit one so we don't have to think about
three presets.

[1] https://github.com/expo/expo/blob/master/packages/jest-expo/README.md#platforms

jest.config.js

@@ -24,9 +24,7 @@ const transformModulesWhitelist = [
 // (This value is correctly a string, not a RegExp.)
 const transformIgnorePattern = `node_modules/(?!${transformModulesWhitelist.join('|')})`;
 
-module.exports = {
-  preset: 'jest-expo',
-
+const common = {
   // Finding and transforming source code.
 
   testPathIgnorePatterns: ['/node_modules/', '/src/__tests__/lib/', '-testlib.js$'],
@@ -49,3 +47,8 @@ module.exports = {
   setupFiles: ['./jest/globalFetch.js', './node_modules/react-native-gesture-handler/jestSetup.js'],
   setupFilesAfterEnv: ['./jest/jestSetup.js', 'jest-extended'],
 };
+
+module.exports = {
+  // See https://github.com/expo/expo/blob/master/packages/jest-expo/README.md#platforms.
+  projects: [{ ...common, displayName: 'ios', preset: 'jest-expo/ios' }],
+};