Add HTTP/2 support to the Go test server

The HTTPS listener will now also serve HTTP/2 requests.

Change-Id: I75c2d4e78ad56a6338f63073f13655a290353ec2
diff --git a/octavia_tempest_plugin/contrib/test_server/test_server.go b/octavia_tempest_plugin/contrib/test_server/test_server.go
index f8bc1e0..726ac5a 100644
--- a/octavia_tempest_plugin/contrib/test_server/test_server.go
+++ b/octavia_tempest_plugin/contrib/test_server/test_server.go
@@ -160,6 +160,7 @@
 				tls.TLS_RSA_WITH_AES_256_GCM_SHA384,
 				tls.TLS_RSA_WITH_AES_256_CBC_SHA,
 			},
+			NextProtos: []string{"h2", "http/1.1", "http/1.0"},
 		}
 	}
 	tls_config.Rand = rand.Reader