Introduce generated unittests for REST API calls generated by ^%REST vs Open API spec

^%REST can generate REST API classes that handle calls listed in Open API spec.

It'd be great if the lib could also generate unittests for all the implementation methods.

Tests could leverage approach, provided by David Hockenbroch here, which helps to setup %request environment for a given REST API method:

set %request = ##class(%CSP.Request).%New() set %request.ContentType = "application/json" set %request.Method = "POST" set %request.Content = ##class(%CSP.CharacterStream).%New() set json = {}.%New() set json.firstname = "David" set json.lastname = "Hockenbroch" do %request.Content.Write(json.%ToJSON())