In my case, there are db tables with users and their properties. In users table got login field (Domain/UserName). Also got custom Identity and Principal, which using this table to authenticate user in global.asax autherticate_request method.
To simulate different users in debug mode, there is special control rendered in master page (or layout page for mvc). It contain select box with all users, and apply button.
On server side after apply button click in session pushed emulated user login and sending refresh to response.
So in next authenticate event occured checking for emulated user login in session and if it exists then authenticate continues based on emulateded login.
Bad side: need sync service to always have actual user table based on current AD.