fix(test): close WebSocket connection in Echo handler

This _should_ fix flaky test `TestSentReceivedMetrics`.
See https://circleci.com/gh/loadimpact/k6/6474, https://circleci.com/gh/loadimpact/k6/6484 .

This issue was introduced in a63bb58 (PR #1138), where the previous
implementation of `getWebsocketEchoHandler()` closed the connection
after writing, so this change brings that back.

It seems that closing the connection creates additional metrics which
`TestSentReceivedMetrics` takes into account _sometimes_, hence the
flakiness.

From discussions with @na--, we agreed to remove the persistent
connection implementation of a63bb58 (the `for` loop here) since that
matches the previous version, but we'll create a new WS test that
tests multiple message passing, since none of our current tests seem
to do this.

lib/testutils/httpmultibin/httpmultibin.go

@@ -106,15 +106,17 @@ func websocketEchoHandler(w http.ResponseWriter, req *http.Request) {
 		return
 	}
 
-	for {
-		mt, message, err := conn.ReadMessage()
-		if err != nil {
-			break
-		}
-		err = conn.WriteMessage(mt, message)
-		if err != nil {
-			break
-		}
+	mt, message, err := conn.ReadMessage()
+	if err != nil {
+		return
+	}
+	err = conn.WriteMessage(mt, message)
+	if err != nil {
+		return
+	}
+	err = conn.Close()
+	if err != nil {
+		return
 	}
 }