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

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

Committed: https://code.google.com/p/gyp/source/detail?r=1708

[Nico, 2013-09-03 18:03:49]

`find test -name 'gyptest*' | grep -v .py` finds one other case, which is broken
by now. I'm trying to get that fixed, and then I'll probably teach the test
runner to error out on single tests that aren't named 'gyptest*.py'.

[Mark Mentovai, 2013-09-03 18:04:50]

LGTM

[Nico, 2013-09-03 18:09:44]

Committed patchset #1 manually as r1708 (presubmit successful).