Add tests for module namespace objects

[jugglinmike]

Assert correct behavior of the own properties of module namespace objects and the essential internal methods of module namespace exotic objects.

In the process of writing these tests, I identified two potential improvements to the specification:

• gh-507: [editorial] Assert normal completion
  value
• gh-510: [editorial] Specify order of module namespace
  keys

Although those patches are still under review, I wrote this changeset under the assumption that they are correct. If I was mistaken, I will update the tests here as appropriate.

[goyakin]

@boingoing & @Yongqu, could you take a look?