Minor Dialyzer type fixes for Riak CS

This doesn't actually get rid of a warning ... though I really did try, using this spec from "typer" and then editing to add the 'test' atom as possible for the last arg:

    %% Spec to avoid "'testing' can never match the type {'wm_reqdata'..."
    -spec try_path_binding(maybe_improper_list(),[nonempty_string()],[[any(),...]],integer(),[{_,_}],0 | 1,'testing' | {'wm_reqdata',_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_}) -> {'no_dispatch_match',[[any(),...]]} | {_,_,[[any(),...]],[{_,_}],string(),string()}.

... but it didn't work. So I'm changing the used-by-eunit-tests-only atom to make it clear that this is an ignorable warning.