Attempt to fix test_stdin_broken_pipe on Travis (#7210)

Author: 1st1

Lib/test/test_asyncio/test_subprocess.py

@@ -228,6 +228,7 @@ def test_stdin_broken_pipe(self):
         proc, large_data = self.prepare_broken_pipe_test()
 
         async def write_stdin(proc, data):
+            await asyncio.sleep(0.5, loop=self.loop)
             proc.stdin.write(data)
             await proc.stdin.drain()