test: don't close connection on write error

The echo server shouldn't close the connection when there's an error on
write. Instead simply echo the error message and allow the other side to
close the connection.

Also do a partial revert of 4d905fb where after_shutdown was removed.

Fixes: 4d905fb "test: close stream immediately on error"
Signed-off-by: Trevor Norris <trev.norris@gmail.com>
Signed-off-by: Saúl Ibarra Corretgé <saghul@gmail.com>