Update testing.rst
Update testing.rst

It now describes how to install flaskr using pip so that all tests pass.
pallets/flask,drewja/flask,fkazimierczak/flask,mitsuhiko/flask,pallets/flask,fkazimierczak/flask,fkazimierczak/flask,pallets/flask,drewja/flask,mitsuhiko/flask,drewja/flask