py3: fix test-bookmarks-corner-case.t

For some reasons, the output of print was not going through. Replaced that
ui.status().

Differential Revision: https://phab.mercurial-scm.org/D6481

contrib/python3-whitelist

@@ -36,6 +36,7 b' test-bisect2.t'
 test-bisect3.t
 test-blackbox.t
 test-bookflow.t
+test-bookmarks-corner-case.t
 test-bookmarks-current.t
 test-bookmarks-merge.t
 test-bookmarks-pushpull.t

tests/test-bookmarks-corner-case.t

@@ -122,7 +122,7 b' We build a server side extension for thi'
   > from mercurial import bookmarks, error, extensions
   > def wrapinit(orig, self, repo):
   >     if not os.path.exists('push-A-started'):
-  >         print('setting raced push up')
+  >         repo.ui.status(b'setting raced push up\n')
   >         with open('push-A-started', 'w'):
   >             pass
   >     clock = 300