bug 851278 - exclude test stub apps from dogfood builds

(In reply to Jason Smith [:jsmith] from comment #8)
> (In reply to Fabrice Desré [:fabrice] from comment #7)
> > external-dogfood-apps should be in anytime DOGFOOD=1 (see
> > https://github.com/mozilla-b2g/gaia/blob/master/build/utils.js#L178)
> > 
> > Would moving them to "test" apps make everyone happy? (I would be)
> 
> Yeah, that works.

Ok, here's a change that does that.  All it does is move the files from *external-dogfood-apps/* to *test_external_apps/*, which causes the build system to build them only when the *DOGFOOD* and *PRODUCTION* variables are set to *'0'* in *build/utils.js*.