run-tests: replace '/dev/null' with os.devnull for Windows
Many of the *.py tests were dying on:
File "c:\Users\Matt\projects\hg\hgdemandimport\tracing.py", line 27, in log
_pipe = open(os.environ['HGCATAPULTSERVERPIPE'], 'w', 1)
IOError: [Errno 2] $ENOENT$: '/dev/null'
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program