Doc Improvements for running test-suite

[mabruzzo]

After helping someone with running Grackle's test suite, I had 2 suggestions to make the documentation friendlier for people who have never been involved in python code development (or have never run tests for a python package):

1. On the Running the Test Suite page of the documentation, it might be helpful to emphasize that there are extra dependencies that need to be manually installed to run the test suite. Maybe we can replace

   Once you have installed pygrackle, the tests can be run from the src directory by typing make test:

   with

   Once you have installed pygrackle (and manually installed the extra test-dependencies), the tests can be run from the src directory by typing make test:

   Back when I first got involved with Grackle, I remembering being a little surprised that things like flake8 and pytest are not automatically installed (to be clear, I think the current way we do it makes a lot of sense to do it this way, but I still initially found it unintuitive). I know that we tell people they need to install these packages on the Pygrackle page, but it can be easy to miss (I certainly did when I first got started).

2. On the you have installed pygrackle webpage, we should probably replace occurrences of py.test with pytest (the latter is the name that is listed on PYPI & on their website)