Normalize _unittest.cc filename suffix for unittests

This allows tools that use filename heuristics to detect tests
(like scythe)

BUG=335193

Review URL: https://codereview.chromium.org/138433004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245392 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245392