use of atexit is fragile - breaks under trial

[Robert Collins]

bzrlib's selftest is using atext for some strange reason.

 affects bzr
 status confirmed
 importance high
 tag selftest

Anyhow...
running tests with trial, dies at the end. It appears to be registering
the same helper multiple times.

PASSED (successes=59)
Error in atexit._run_exitfuncs:
Traceback (most recent call last):
  File "/usr/lib/python2.6/atexit.py", line 24, in _run_exitfuncs
    func(*targs, **kargs)
  File "/usr/lib/python2.6/dist-packages/bzrlib/tests/__init__.py", line
3774, in _rmtree_temp_dir
    osutils.rmtree(dirname)
  File "/usr/lib/python2.6/shutil.py", line 208, in rmtree
    onerror(os.listdir, path, sys.exc_info())
  File "/usr/lib/python2.6/shutil.py", line 206, in rmtree
    names = os.listdir(path)
OSError: [Errno 2] No such file or directory: '/tmp/testbzr-YPWZ_8.tmp'