example-test.js

jest.dontMock('../example');

describe('example', function() {
  it('does stuff', function() {
    var underscore = require('underscore');
    console.log('underscore', underscore.toString());
  });
});

example.js

var underscore = require('underscore');

console.log(underscore);

package.json

{
  "name": "jefftest",
  "devDependencies": {
    "jest-cli": "^0.1.2"
  },
  "dependencies": {
    "underscore": "^1.6.0"
  },
  "scripts": {"test": "jest"},
  "jest": {
    "unmockedModulePathPatterns": [
      "<rootDir>/node_modules/underscore"
    ]
  }
}