Better protections for typing-extensions tests

[JelleZijlstra]

Followup from python/typing#935:

• Add assertion to make sure the typing-extensions we're testing is the one in the repo, not the one that happens to be installed in your venv.
• Document how to test typing-extensions locally (probably, we should recommend using an empty venv).

[JelleZijlstra]

I submitted documentation on how to run tests in #211.

I thought about adding a test in test_typing_extensions.py that asserts that the typing_extensions it's using is in the same directory as the current file, but I feel that sort of thing may cause trouble for redistributors later, so I'm inclined to skip that idea and close this issue after the documentation change is merged.