Fix Jest mocks for Dimensions module

Summary:
Jest mock for Dimensions.get("screen") is missing. Tests fail with error message:

    Invariant Violation: No dimension set for key screen

Run test with usage of Dimensions.get("screen")

[GENERAL] [BUGFIX] [jest/setup.js] - Fix Jest mocks for Dimensions module
Closes https://github.com/facebook/react-native/pull/17017

Differential Revision: D6471719

Pulled By: TheSavior

fbshipit-source-id: de27b2819d088fefe20dfca94ab5672fdec5f0e1

jest/setup.js

@@ -142,6 +142,12 @@ const mockNativeModules = {
         scale: 2,
         width: 750,
       },
+      screen: {
+        fontScale: 2,
+        height: 1334,
+        scale: 2,
+        width: 750,
+      },
     },
   },
   FacebookSDK: {