Sign in as different user is missing in SharePoint 2013. It will be very difficult for testing scenarios with different users. I use this functionality very frequently. I guess Microsoft wants that user don’t hack the site by typing wild guessing the passwords.
There are three options to sign in as different user
1) Just go to start->programs->IE and shift-right click and “run as different user”
2) a) Locate and then open the following file in a text editor: C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\TEMPLATE\CONTROLTEMPLATES\Welcome.ascx and add the following element before the existing “ID_RequestAccess” element: