test: http2 connectionListener reject client

This code change modifies connectionListener tests to cover test case
where this.emit('unknownProtocol', socket) returns false

PR-URL: nodejs#16080
Ref: nodejs#14985
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-http2-https-fallback.js

@@ -110,9 +110,9 @@ function onSession(session) {
     common.mustCall(onRequest)
   );
 
-  server.on('unknownProtocol', common.mustCall((socket) => {
+  server.once('unknownProtocol', common.mustCall((socket) => {
     socket.destroy();
-  }, 2));
+  }));
 
   server.listen(0);