[loader] Remove leftover `test` config from patterns

Patterns are implemented so that the module object that is added to the Loader through `addModule` is the pattern object itself. This means that `getModule('foo-that-matches-pattern')` will always return an object with a `test` function. But this test function that not represent a test over the module, but a pattern test, and it's a conflict with our future optional dependencies feature.

This PR just aims to leave the door open for `config.test` to be something other than a pattern function.

Another possibility would be to use `Y.mix({}, found, false, [some whitelist])` but I'm not sure what the whitelist would look like. If you think using `mix` makes more sense, please speak up and help me figure out what the whitelist should be. Otherwise I think this quick solution should be good enough.

/cc @caridy