ZulipAsyncStorage: Add a test for setItem.

With a new mock for `TextCompressionModule`; we do that globally in
case it's useful elsewhere. That module should grow its own set of
unit tests.

jest/jestSetup.js

@@ -45,6 +45,13 @@ jest.mock('react-native', () => {
       resourceURL:
         'file:///private/var/containers/Bundle/Application/4DCD4D2B-F745-4C70-AD74-8E5F690CF593/ZulipMobile.app/',
     };
+  } else if (ReactNative.Platform.OS === 'android') {
+    const header = 'z|mock|';
+    ReactNative.NativeModules.TextCompressionModule = {
+      header,
+      compress: async (input: string) => `${header}${input}`,
+      decompress: async (input: string) => input.replace(header, ''),
+    };
   }
 
   return ReactNative;

src/boot/__tests__/ZulipAsyncStorage-test.js

@@ -0,0 +1,33 @@
+/* @flow strict-local */
+import { NativeModules, Platform } from 'react-native';
+import AsyncStorage from '@react-native-community/async-storage';
+
+import ZulipAsyncStorage from '../ZulipAsyncStorage';
+
+// Some of these tests here are written to differ slightly between iOS
+// and Android, which might make them a bit harder to understand. But
+// most of the logic is appropriately in common between the platforms,
+// so we haven't split the different logic up into different tests.
+//
+// We may in the future decide to add (or replace this file with) a
+// `ZulipAsyncStorage-test.ios.js` and
+// `ZulipAsyncStorage-test.android.js`, to target entire test suites
+// to Android and iOS.
+describe('ZulipAsyncStorage', () => {
+  describe('setItem', () => {
+    const setItemSpy = jest.spyOn(AsyncStorage, 'setItem');
+
+    const key = 'foo!';
+    const value = '123!';
+    const callback = err => {};
+
+    test('calls AsyncStorage.setItem as we expect it to', async () => {
+      await ZulipAsyncStorage.setItem(key, value, callback);
+
+      const expectedValue =
+        Platform.OS === 'ios' ? value : await NativeModules.TextCompressionModule.compress(value);
+      expect(setItemSpy).toHaveBeenCalledTimes(1);
+      expect(setItemSpy).toHaveBeenCalledWith(key, expectedValue, callback);
+    });
+  });
+});