It doesn't matter which order test_os and test_ctypes run in, but if they precede test_wait3 test_wait3 will fail.  When the test is immediately re-run it passes.

The reason ought to be interesting once we figure it out :)

Tested on linux.  Doesn't appear to affect other branches.

[1/3] test_os
[2/3] test_ctypes
[3/3] test_wait3
test test_wait3 failed -- Traceback (most recent call last):
  File "/home/rdmurray/python/p33/Lib/test/fork_wait.py", line 72, in test_wait
    self.wait_impl(cpid)
  File "/home/rdmurray/python/p33/Lib/test/test_wait3.py", line 30, in wait_impl
    self.assertEqual(spid, cpid)
AssertionError: 14480 != 14487

What seems to happen is that test_ctypes runs a lot of ldconfig processes, and the 10 retries in fork_wait are not enough to consume them all.

New changeset 9e473917cbfb by Antoine Pitrou in branch 'default':
Issue #12041: Make test_wait3 more robust.
http://hg.python.org/cpython/rev/9e473917cbfb

This should be fixed, can you confirm?

PS: I've opened issue12044 as one of the root causes.

Works for me after the commit.

Confirmed working on my box as well, and on my buildbot (where I first noticed it).