python -m psutil.tests doesn't work

[giampaolo]

~/svn/psutil {master}$ python -m psutil.tests
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/giampaolo/svn/psutil/psutil/tests/__main__.py", line 96, in <module>
    main()
  File "/home/giampaolo/svn/psutil/psutil/tests/__main__.py", line 93, in main
    run_suite()
  File "psutil/tests/__init__.py", line 764, in run_suite
    _setup_tests()
  File "psutil/tests/__init__.py", line 747, in _setup_tests
    assert 'PSUTIL_TESTING' in os.environ
AssertionError