Add Prereq section to Testing page

[jkunkee]

From just trying to run them, the tests seem to be Python-3-only, so I added this fact to the Testing page.

I have only confirmed the part about the test telling you if something is missing with MSVC.

[refack]

I am trying to keep Python2 compatibility even for the tests - https://travis-ci.com/refack/GYP/jobs/181771177
Admittedly ATM we only have a Python3 windows CI config, But localy I run python2 gyptest.py -a -f msvs-mock.

P.S. The docs do seems a bit dated in other respects as well :SMH:

[refack]

Suggested change

	* Python 3 in the path
	* Python 3 (or 2) in the path
	* Tests should pass on both Python3.7 and Python2.7

[refack]

Suggested change

* Using python 2 causes "only strings in environment" errors