Rework and comment the test-cases for pkgutil.extend_path()

This is a follow up to #1283 (see https://github.com/pyinstaller/pyinstaller/issues/1283#issuecomment-131061632).

The python3-branch supports pkgutil.extend_path(). But the test-cases (import/test_nspkg3*.py).work only "basically", because pkgutil.extend_path() behaves different from the other namespace-techniques if zipped eggs are involved. And this is the case in the current test-case. So we need to rework and comment the test-cases.

One thing we need to keep in mind for the test-cases (from pydoc pkgutil.extend_path):

Items of sys.path that are not (unicode or 8-bit) strings referring to existing directories are ignored

This has another implication: Any a zipped egg (even if using pkgutil.extend_path()) will be kicked out of the namespace's __path__, as soon as there is another packages for this namespace in sys.path which is using pkgutil.extend_path()