Provide a way to unit test controller redirects unaware of url mappings

In Grails 3.x, currently we can acess response.redirectedUrl in a unit test to check for a controller redirection, but the returned url is affected by url mappings, which one may feel unconfortable with while testing the controller

Instead we could have something like controller.redirectArgs which would return a map with the exact arguments (unaffected by url mappings) that were used inside the controller to do the redirection

Start by examining Grails 3.x controller unit-test redirect handling, especially how response.redirectedUrl is produced and affected by URL mappings. Define the redirect arguments exposed by the proposed controller.redirectArgs API, then add unit coverage showing they remain unchanged by URL mappings.

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