Assign Jetty SockJS tests to "performance" test group

This change designates Jetty SockJS integration tests to run as part of
the "performance", but not the main "publication", CI build due to
recurring low-level failures suspected to be Jetty issues, e.g.
"java.io.IOException: Cannot append to finished buffer" or
"java.io.IOException: Out of order Continuation frame encountered".

The tests will still run at once a day with the performance build but
should not fail the main build with false negatives. Also note that
an Undertow variant of the exact same tests, which hasn't been failing,
will continue to run as part of the main build.

spring-websocket/src/test/java/org/springframework/web/socket/sockjs/client/JettySockJsIntegrationTests.java

@@ -17,9 +17,11 @@
 package org.springframework.web.socket.sockjs.client;
 
 import org.eclipse.jetty.client.HttpClient;
-
+import org.junit.BeforeClass;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.tests.Assume;
+import org.springframework.tests.TestGroup;
 import org.springframework.web.socket.JettyWebSocketTestServer;
 import org.springframework.web.socket.client.jetty.JettyWebSocketClient;
 import org.springframework.web.socket.server.RequestUpgradeStrategy;
@@ -32,6 +34,10 @@ import org.springframework.web.socket.server.jetty.JettyRequestUpgradeStrategy;
  */
 public class JettySockJsIntegrationTests extends AbstractSockJsIntegrationTests {
 
+	@BeforeClass
+	public static void setUpOnce() throws Exception {
+		Assume.group(TestGroup.PERFORMANCE);
+	}
 
 	@Override
 	protected Class<?> upgradeStrategyConfigClass() {