win: Fix behaviour of MinimumRequiredVersion

Previous change for ninja set default subsystem to XP, but didn't do
anything for msvs. This broke the 2012 bot which is where the default
changed (I thought it was 2013 previously).

Now, use the MinimumRequiredVersion setting to determine this value,
which works in VS >= 2010 and emulate that in ninja.

Also, add another line to TestGyp.py so that gyptest.py works when
running on VS2013.

R=thakis@chromium.org
BUG=320371

Review URL: https://codereview.chromium.org/67373005

git-svn-id: http://gyp.googlecode.com/svn/trunk@1788 78cadc50-ecff-11dd-a971-7dbc132099af