jest: Mock "Linking" the right way.

In the upcoming RN v0.60 -> v0.61 upgrade, Haste won't be used, so
the string "Linking" won't be resolved.

Move our "Linking" mock into our mock of 'react-native', added
in a recent commit, rather than pointing to
'react-native/Libraries/Linking/Linking' and thus coupling our tests
with the internal directory structure of `react-native`. [1]

[1]: See React Native's recommendation of this strategy, at
     facebook/react-native#26579 (comment).

jest/jestSetup.js

@@ -43,6 +43,14 @@ jest.mock('react-native', () =>
       Platform: ReactNative.Platform,
       StyleSheet: ReactNative.StyleSheet,
       NativeModules: ReactNative.NativeModules,
+
+      Linking: {
+        addEventListener: jest.fn(),
+        removeEventListener: jest.fn(),
+        openURL: jest.fn(),
+        canOpenURL: jest.fn(),
+        getInitialURL: jest.fn(),
+      },
     },
     ReactNative,
   ),
@@ -54,14 +62,6 @@ jest.mock('react-native-sound', () => () => ({
   play: jest.fn(),
 }));
 
-jest.mock('Linking', () => ({
-  addEventListener: jest.fn(),
-  removeEventListener: jest.fn(),
-  openURL: jest.fn(),
-  canOpenURL: jest.fn(),
-  getInitialURL: jest.fn(),
-}));
-
 jest.mock('rn-fetch-blob', () => ({
   DocumentDir: () => {},
 }));