spec.js

describe("MyTest", function() {
  
  var Subject;
  
  beforeEach(function(done) {
    this.stub({ "stubbed_module": {});
    this.require(["module_to_test"], function(s) {
      Subject = s;
      done();
    });
  });
  
  it("test it");
  
})

spec_helper.js

beforeEach(function() {
  this.req = null;
  this.map = {};
  this.stubs = {};

  this.stub = function(stubs) {
    var self = this;

    for (var key in stubs) {
      var stubModule = "stub_" + key;

      this.map[key] = stubModule;
      this.stubs[stubModule] = stubs[key];

      if (require.defined(stubModule)) {
        require.undef(stubModule);
      }

      define(stubModule, function() {
        return self.stubs[stubModule]; 
      });
    }
  };

  // ### require
  // this function provides a local "require" that is unique
  // for each test. Meaning that you can provides
  this.require = function() {
    var self = this;
    var deps = arguments[0];
    var callback = arguments[1];

    if (!this.req) {
      // if require hasn't been used for this test
      // context yet, create it.
      var context = Math.floor(Math.random() * 1000000);
      this.req = require.config({
        baseUrl: '/lib/app',
        context: context,
        map: {
          '*': self.map
        }
      });
    }

    this.req(deps, function() {
      var args = arguments;
      callback.apply(self, arguments);
    });
  };
});