Support 'msvs_prebuild/msvs_postbuild' in MSBuild (VS 2010 and later)

Originally by yukawa_g <yukawa@google.com>,
see <https://codereview.chromium.org/8229003>, except that I moved the
test to the test directory and renamed the gyptest file; and added a
warning to the ninja generator that it doesn't support these.

Patch from stk.haller@googlemail.com.

R=bradnelson@google.com

Review URL: https://codereview.chromium.org/137853003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1837 78cadc50-ecff-11dd-a971-7dbc132099af