Move TestNestscapePlugIn from DumpRenderTree to testing/plugin.

This is a continuation of the work to remove Tools/DumpRenderTree/ now that
we have content_shell.

BUG=None
R=abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22079002

git-svn-id: svn://svn.chromium.org/blink/trunk@155487 bbb929c8-8fbe-4397-9dbb-9b2b20218538