Move TestRunner API headers to public/testing

This CL moves the last block of API headers to the top-level public directory.
Moving these headers makes even more sense than the other headers because
there's no such thing as DumpRenderTree anymore.

BUG=239545
R=jochen

Review URL: https://chromiumcodereview.appspot.com/17155027

git-svn-id: svn://svn.chromium.org/blink/trunk@152931 bbb929c8-8fbe-4397-9dbb-9b2b20218538