Override the user's language settings during tests

If a user's language is set to anything other than English, the output
of various tools may vary from what is expected and cause tests to
fail. This fixes several test failures for non-English users.

To reproduce the issue, run: LC_ALL=de_DE.UTF-8 ./gyptest.py -a

Patch by Joey Parrish <joeyparrish@google.com>

Review URL: https://codereview.chromium.org/324893007/


git-svn-id: http://gyp.googlecode.com/svn/trunk@1932 78cadc50-ecff-11dd-a971-7dbc132099af