tests: sort some imports that were previously missed

I'm a little unclear why the import checker didn't catch this before,
but when I fixed it to work in Python 3 this failure started showing
up. Sigh.

Differential Revision: https://phab.mercurial-scm.org/D6454

tests/test-bookmarks-corner-case.t

@@ -116,10 +116,10 b' If two process are pushing we want to ma'
 We build a server side extension for this purpose
 
   $ cat > bookrace.py << EOF
+  > import atexit
   > import os
   > import time
-  > import atexit
-  > from mercurial import error, extensions, bookmarks
+  > from mercurial import bookmarks, error, extensions
   > def wrapinit(orig, self, repo):
   >     if not os.path.exists('push-A-started'):
   >         print('setting raced push up')