tests: Use os.devnull instead of hardcoded '/dev/null'.
tests: Use os.devnull instead of hardcoded '/dev/null'.

Patch written by Serhiy Storchaka <storchaka@gmail.com>.

fallen/asyncio,jashandeep-sohi/asyncio,Martiusweb/asyncio,gvanrossum/asyncio,gsb-eng/asyncio,manipopopo/asyncio,gsb-eng/asyncio,gvanrossum/asyncio,manipopopo/asyncio,gvanrossum/asyncio,Martiusweb/asyncio,ajdavis/asyncio,1st1/asyncio,jashandeep-sohi/asyncio,1st1/asyncio,manipopopo/asyncio,haypo/trollius,haypo/trollius,Martiusweb/asyncio,ajdavis/asyncio,vxgmichel/asyncio,1st1/asyncio,jashandeep-sohi/asyncio,haypo/trollius,vxgmichel/asyncio,vxgmichel/asyncio,gsb-eng/asyncio,fallen/asyncio,fallen/asyncio,ajdavis/asyncio