Fixed memory leak of IHub instances. This resolves #3208.  DefaultDepend...

...encyResolver will now avoid storing IHub instances in _trackedDisposables.

- Unit test included.  Without the fix, the test will fail.
- I could not locate an existing file/class for DefaultDependencyResolver tests, so I created a new one. Please advise if not correct.
- This fix needs to also go into 2.1.x and 1.2.x.  I can create multiple PRs for that if needed. Please advise.
Nit: Might as well make all these test classes private.