test-check-code: fail new commits which use 'atexit' instead of 'ui.atexit' Callbacks registered with 'atexit' are sometimes not called (like when hg aborts and calls os._exit). On the other hand, callbacks registered with 'ui.atexit' are called in most cases. Therefore, encouraging the use of 'ui.atexit' by failing the test 'test-check-code.t' appropriately. Test Plan: Ran the test 'test-check-code.t' after importing 'atexit' in one of the py files and confirmed that the test fails. Differential Revision: https://phab.mercurial-scm.org/D961