How do you retrieve the current 'simpletest' user during a simpletest.
During a simpletest is the $user GLOBAL from the test environment or the live environment?
Inside the test method?
No idea what in $GLOBALS['user'] is, but you certainly can't rely on it. You are never "logged in" in that context, you are only logged in when doing requests with drupalGet()/drupalPost().
If you used drupalLogin(), you can look for $this->loggedInUser. If not, then it's not possible, other than maybe doing a GET to /user and then trying to get the username from title.
Inside a page callback, $GLOBALS['user'] is the current user, just like you'd expect.