Mojo: Add a public/environment/test_support.h.

Remove public/tests's base dependency.

R=darin@chromium.org

Review URL: https://codereview.chromium.org/145073008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250439 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250439