Type mock.contexts correctly in Jest (#37847)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/37847

Jest supports accessing the contexts used to call mock functions (the `this` value) but we're not typing that correctly in the definitions in React Native. This changes that.

Changelog: [internal]

Reviewed By: javache

Differential Revision: D46684043

fbshipit-source-id: ff121e0726748788591e7d061a39fde238c1ddbe

packages/react-native/flow/jest.js

@@ -38,6 +38,10 @@ type JestMockFn<TArguments: $ReadOnlyArray<any>, TReturn> = {
      * instantiated from this mock function.
      */
     instances: Array<TReturn>,
+    /**
+     * An array that contains the contexts for all calls of the mock function.
+     */
+    contexts: Array<mixed>,
     /**
      * An array that contains all the object results that have been
      * returned by this mock function call