[DOC] Add steps for running tests on Windows

[gmarkall]

The documentation for running tests in the readme only describes the process on Linux.

We should also add the equivalent steps for running tests on Windows.

[acosmicflamingo]

Before writing the equivalent documentation for Windows, perhaps the first thing to discuss is whether we want to require users to have a Windows-equivalent make installed or whether we should maintain a separate Makefile.win for them.

The latter will certainly be more beneficial for Windows users since we don't have to require them to install yet another dependency. But, if most (if not all) numba-cuda users on the Windows platform already have a Windows-equivalent make installed (e.g. msys2, cygwin, etc.), then perhaps this is a win for maintainability since now we just have to worry about updating one single Makefile.