Add testing harness that replicates FastBoot environment.

[rwjblue]

Prior to this we were simulating FastBoot in a non-isolated VM context. That meant that the running Node context was available within rendering attempts. However, when ran with "real" FastBoot that is not the case. In the real FastBoot environment, we create an isolated vm context and evaluate both Ember and the application within that context. This ensures that the running context does not have access to arbitrary Node-land APIs (e.g. require).

This replicates the failure that is reported in glimmerjs/glimmer-vm#1141:

  message: "beforeEach failed on FastBoot: basic: Cannot read property 'require' of null"
  severity: failed
  actual: null
  expected: undefined
  stack: /Users/rjackson/src/emberjs/ember.js/dist/tests/ember.js:46731
    var nodeURL = _nodeModule.module.require('url');
                                     ^

TypeError: Cannot read property 'require' of null
    at /Users/rjackson/src/emberjs/ember.js/dist/tests/ember.js:46731:38
    at internalRequire (/Users/rjackson/src/emberjs/ember.js/dist/tests/ember.js:64:14)
    at internalRequire (/Users/rjackson/src/emberjs/ember.js/dist/tests/ember.js:60:22)
    at internalRequire (/Users/rjackson/src/emberjs/ember.js/dist/tests/ember.js:60:22)
    at internalRequire (/Users/rjackson/src/emberjs/ember.js/dist/tests/ember.js:60:22)
    at require (/Users/rjackson/src/emberjs/ember.js/dist/tests/ember.js:97:14)
    at /Users/rjackson/src/emberjs/ember.js/dist/tests/ember.js:64647:1
    at /Users/rjackson/src/emberjs/ember.js/dist/tests/ember.js:64648:2
    at Script.runInContext (vm.js:142:18)
    at buildSandboxContext (/Users/rjackson/src/emberjs/ember.js/tests/node/fastboot-sandbox-test.js:86:15)