Update testing.rst

Motivation:
`python setup.py nosetests` requires requirements.txt be installed, the `-e '.[paging]' --use-mirrors` dependencies, and this should both be done inside a venv.
make test-install does that.

Also adding a guide for running single unit tests so that the dependencies isn't a mystery. Running `python setup.py nosetests` by itself returns errors that aren't immediately clear that nosetests needs to be pip installed (like an import error).