The atexit module effectively turned itself off if sys.exitfunc already

The atexit module effectively turned itself off if sys.exitfunc already
existed at the time atexit first got imported.  That's a bug, and this
fixes it.

Also reworked test_atexit.py to test for this too, and to stop using
an "expected output" file, and to test what actually happens at exit
instead of just simulating what it thinks atexit will do at exit.

Bugfix candidate, but it's messy so I'll backport to 2.2 myself.