Update unittest_gtest.py

Change from current version is:

* Upgrade to gtest-1.7.
* Print stderr for failed tests, as proposed in https://github.com/tanakh/waf-unittest/pull/9 (fixes #726).

FYI, difference between gtest-1.6 and gtest-1.7 is summarized in:
* http://srz-zumix.blogspot.jp/2013/09/google-test-170-rc.html
* http://srz-zumix.blogspot.jp/2013/09/google-test-170.html