Why does it need to be a valid loader?  It's a mock, not a real 
loader.  But if it really bothers you, have it increment a global in 
the module or something, and put the assertion in the test 
proper.  Heck, have it update a counter in the module it returns, 
making it into a loader that keeps track of how many times you reload 
the same module.  Then it's a useful example loader.  :)