tests/misc/sys_atexit: Add test for new sys.atexit feature.

tests/misc/sys_atexit.py

+# test sys.atexit() function
+
+import sys
+try:
+    sys.atexit
+except AttributeError:
+    print('SKIP')
+    raise SystemExit
+
+some_var = None
+
+def do_at_exit():
+    print("done at exit:", some_var)
+
+sys.atexit(do_at_exit)
+
+some_var = "ok"
+print("done before exit")

tests/misc/sys_atexit.py.exp

+done before exit
+done at exit: ok