Add test for the "generate mixed-case header" bug.

This test failed on Windows with ninja 1.3.3 but
should pass with ninja 1.3.4 which has
https://github.com/martine/ninja/commit/02d51b3f3d3058411a06bb08d5b773f60848fec4

BUG=chromium:242397
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/16050013

git-svn-id: http://gyp.googlecode.com/svn/trunk@1645 78cadc50-ecff-11dd-a971-7dbc132099af