conn_test.go: Fix TestConcurrentClosePing

Closes #298
Closes #394

The close frame was being received from the peer before we were able to reset our
write timeout and so we thought the write kept failing but it never was...

Thanks @univerio and @bhallionOhbibi

Author: nhooyr

Diff for: read.go

@@ -62,9 +62,12 @@ func (c *Conn) Read(ctx context.Context) (MessageType, []byte, error) {
 func (c *Conn) CloseRead(ctx context.Context) context.Context {
 	ctx, cancel := context.WithCancel(ctx)
 	go func() {
+		defer c.CloseNow()
 		defer cancel()
-		c.Reader(ctx)
-		c.Close(StatusPolicyViolation, "unexpected data message")
+		_, _, err := c.Reader(ctx)
+		if err == nil {
+			c.Close(StatusPolicyViolation, "unexpected data message")
+		}
 	}()
 	return ctx
 }

Diff for: write.go

@@ -323,6 +323,9 @@ func (c *Conn) writeFrame(ctx context.Context, fin bool, flate bool, opcode opco
 
 	select {
 	case <-c.closed:
+		if opcode == opClose {
+			return n, nil
+		}
 		return n, errClosed
 	case c.writeTimeout <- context.Background():
 	}