Add Geolocation API Jest mock

Summary:
Hello!

The [Geolocation API](https://facebook.github.io/react-native/docs/geolocation.html) is currently not mocked in Jest, which often leads to an `Invariant Violation: Native module cannot be null` error. This patch adds a basic Jest configuration similar to the existing ones for the other modules.

None unfortunately, but this patch makes my test suite all green 😉

Thanks,
William
Closes https://github.com/facebook/react-native/pull/13442

Differential Revision: D4883830

Pulled By: javache

fbshipit-source-id: c2a976834cca7537bd832a698e8cd25cf877705b

jest/setup.js

@@ -131,6 +131,11 @@ const mockNativeModules = {
     addListener: jest.fn(),
     removeListeners: jest.fn(),
   },
+  LocationObserver: {
+    getCurrentPosition: jest.fn(),
+    startObserving: jest.fn(),
+    stopObserving: jest.fn(),
+  },
   ModalFullscreenViewManager: {},
   Networking: {
     sendRequest: jest.fn(),