test-push-race: use 'ui.atexit' instead of python's 'atexit'

Callbacks registered with 'atexit' are sometimes not called (like when a
process is killed by the SIGTERM signal). Therefore, this commit replaces it
with 'ui.atexit' which ensures that the callbacks are always called. This also
makes the test compatible with chg.

Test Plan:
Ran the test 'test-push-race.t' with and without the '--chg' option.

Differential Revision: https://phab.mercurial-scm.org/D957

tests/test-push-race.t

@@ -15,7 +15,6 b' A set of extension and shell functions e'
   > Client with the extensions will create a file when ready and get stuck until
   > a file is created."""
   > 
-  > import atexit
   > import errno
   > import os
   > import time
@@ -51,7 +50,7 b' A set of extension and shell functions e'
   >                 except OSError as exc:
   >                     if exc.errno != errno.ENOENT:
   >                         raise
-  >             atexit.register(delete)
+  >             ui.atexit(delete)
   >     return orig(pushop)
   > 
   > def uisetup(ui):