Tests: Rename a gyptest*.gyp file to gyptest*.py.

The test runner silently runs gyptest*.gyp files if invoked with a single test,
but doesn't run them when asked to run all tests.

This test happens to still pass, so rename it so that the bots will check that
it keeps passing.

R=mark@chromium.org

Review URL: https://codereview.chromium.org/23905003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1708 78cadc50-ecff-11dd-a971-7dbc132099af