Make tests work on Windows

It occurred to me when testing VS builds that the VS Makefile does not have a test task, so I added that. The test runner stuck on Windows, probably due to `wait()` usages causing dead locks on large inputs, so I changed those to use `communicate()` instead, which is promised (by Python’s documentation) to work.

Obviously this needs both `python` (any modern version) and `tidy` in your `PATH` to work. I use [Letzte Bearbeitung’s HTML Tidy for Windows executable](http://www.paehl.com/open_source/?HTML_Tidy_for_Windows).