test: fix test-http2-socket-close.js
The client receives a response from the server and then destroys the connection after waiting 10ms. This waiting time is sometimes not enough so the connection is closed without seeing UV_EOF.
In order to close the connection gracefully, I increased the waiting time to be on the safe side.
Note that this flakiness could have been fixed using end() instead of destroy(), but I didn't want to change the test logic.