Add test for when overriding default resolver

packages/jest-resolve/src/__mocks__/userResolver.js

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+module.exports = function userResolver(path, options) {
+  return require('./module');
+};

packages/jest-resolve/src/__tests__/Resolver-test.js

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+/**
+ * Copyright (c) 2017, Facebook, Inc. All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+'use strict';
+
+const path = require('path');
+
+const nodePaths =
+  (process.env.NODE_PATH ? process.env.NODE_PATH.split(path.delimiter) : null);
+
+describe('Resolve', () => {
+  describe('findNodeModule', () => {
+    it('is possible to override the default resolver', () => {
+
+      jest.mock('../__mocks__/userResolver');
+      const userResolver = require('../__mocks__/userResolver');
+      userResolver.mockImplementation(() => require('./fixtures/module'));
+
+      const Resolver = require('jest-resolve');
+
+      const newPath = Resolver.findNodeModule('test',
+        {
+          basedir: '/',
+          browser: true,
+          extensions: ['js'],
+          moduleDirectory: ['node_modules'],
+          paths: ['/something'],
+          resolver: require.resolve('../__mocks__/userResolver'),
+        }
+      );
+
+      expect(newPath).toBe(require('./fixtures/module'));
+      expect(userResolver.mock.calls[0][0]).toBe('test');
+      expect(userResolver.mock.calls[0][1]).toEqual({
+        basedir: '/',
+        browser: true,
+        extensions: ['js'],
+        moduleDirectory: ['node_modules'],
+        paths: (nodePaths || []).concat(['/something']),
+      });
+    });
+  });
+});

packages/jest-resolve/src/__tests__/fixtures/module.js

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+module.exports = 42;