Delete Source/testing

It's now all in content/shell and subdirectories

BUG=331302
R=tfarina@chromium.org

Review URL: https://codereview.chromium.org/153633002

git-svn-id: svn://svn.chromium.org/blink/trunk@166716 bbb929c8-8fbe-4397-9dbb-9b2b20218538