Make test_writer_callback() more robust.
Make test_writer_callback() more robust.

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