Some stat test updates

[brody4hire]

• Improve description on built-in stat constructor check
• Add async fs stats constructor test
• statSync uid & gid test fixes
  • fix test description
  • remove duplicated Stats constructor check
• Add async test of stat uid & gid - now skipped on "win32" platform for Travis CI to pass

Open questions:

• I noticed that the async stat call gives us different uid & gid values from the statSync call (I removed check of actual values due to differences between my mac and Travis CI). I am not sure if this is relevant or not?
• async stat call does not seem to give us Truthy uid or gid values on Windows ("win32" platform), any explanation?
• I am not sure if I skipped the async test of uid & gid the right way or not? (some candid feedback is requested)