Get currently active 'simpletest' user during test
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.