jest: Fix jestjs/jest#10221 locally.

This is much easier and less invasive than forking React Native, as
we considered in zulip/react-native#5.

The idea comes from Greg on a video call, with a code example
provided by a @testing-library/react-native contributor [1]. Follow
that example, with tweaks for our project -- using camelCase for the
file names, putting the new files in our `jest` directory, and
adding a TODO mentioning that we'd maybe like to get back to using
jest-expo, which wraps React Native's preset.

[1] jestjs/jest#10221 (comment),
    which links to
    sbalay/without_await@64a76486f.

jest.config.js

@@ -24,7 +24,7 @@ const transformModulesWhitelist = [
 const transformIgnorePattern = `node_modules/(?!${transformModulesWhitelist.join('|')})`;
 
 module.exports = {
-  preset: 'react-native',
+  preset: './jest/preset.js',
 
   // Finding and transforming source code.
 

jest/preset.js

@@ -0,0 +1,19 @@
+/**
+ * The preset we're making tweaks to.
+ */
+// TODO: Switch back to jest-expo (i.e., use 'jest-expo/jest-preset')
+// when it has a version that depends on Jest 26, not Jest 25.
+const basePreset = require('react-native/jest-preset');
+
+module.exports = {
+  ...basePreset,
+  setupFiles: [
+    // Until facebook/jest#10221 is resolved, sandwich the faulty
+    // logic in react-native's preset (in which `global.Promise` is
+    // harmfully replaced) with a fix. See
+    // https://github.com/facebook/jest/issues/10221#issuecomment-714335771.
+    require.resolve('./savePromise.js'),
+    ...basePreset.setupFiles,
+    require.resolve('./restorePromise.js'),
+  ],
+};

jest/restorePromise.js

@@ -0,0 +1,3 @@
+// For facebook/jest#10221. After savePromise.js and Jest's setup
+// files have run, restore the natural value of `global.Promise`.
+global.Promise = global.originalPromise;

jest/savePromise.js

@@ -0,0 +1,4 @@
+// For facebook/jest#10221. Before Jest's setup files have run, take
+// note of what the natural value of `global.Promise` is, so we can
+// restore it in restorePromise.js.
+global.originalPromise = Promise;