Docs for running tests need updated path for manage.py

[pydolan]

On https://behave-django.readthedocs.io/en/latest/contribute.html, all commands using manage.py should be updated to tests/manage.py since it is implied that the tests be run from the root of the project.

Note: If one tries to run the tests from within the tests directory, the py.test tests fail and Behave cannot find the steps directory since the Behave config is not defined (since tox.ini is in the parent directory).

[bittner]

Thanks for pointing this out!

• This should be updated to tox -l and tox -e py37-django21 (example). The testing docs are from the early days when we had no Tox in the project yet.
• Installing the dev requirements should be mentioned to be optional. (There's really just tox, everything else is installed automatically during the test runs.)

In short, all references suggesting manual setup of a development environment should be removed or marked optional. All references suggesting to run something other than tox likewise.

[bittner]

That should be clearer now.