Fix detach assert function to account for new behavior in engine 18.06

Signed-off-by: Joffrey F <joffrey@docker.com>

tests/helpers.py

@@ -123,7 +123,12 @@ def assert_cat_socket_detached_with_keys(sock, inputs):
             sock.sendall(b'make sure the socket is closed\n')
     else:
         sock.sendall(b"make sure the socket is closed\n")
-        assert sock.recv(32) == b''
+        data = sock.recv(128)
+        # New in 18.06: error message is broadcast over the socket when reading
+        # after detach
+        assert data == b'' or data.startswith(
+            b'exec attach failed: error on attach stdin: read escape sequence'
+        )
 
 
 def ctrl_with(char):