ApiStructureTest has a lot of logic for validating the settings arrays in PHPUnit. Some of that logic remains specific to the Action API, but a decent part of it should now be part of the ParamValidator library.
My current thinking is to add a "validateSettings()" method parallel to normalizeSettings(), which will will return a MessageValue if validation fails. ApiStructureTest would call this and fail with the appropriate message.
: Or an array of MessageValues? Or make a Message-library version of Status/StatusValue?