Attempt to fix test_stdin_broken_pipe on Travis (GH-7210) (#7211)

(cherry picked from commit 4fadf0c)

Co-authored-by: Yury Selivanov <yury@magic.io>

Author: miss-islington

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()