xds: annotate XdsRoutingLoadBalancerTest with RunWith (#6855)

This is required inside google for all tests.

Author: zhangkun83

xds/src/test/java/io/grpc/xds/XdsRoutingLoadBalancerTest.java

@@ -61,11 +61,14 @@
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
 import org.mockito.ArgumentCaptor;
 import org.mockito.Mock;
 import org.mockito.MockitoAnnotations;
 
 /** Tests for {@link XdsRoutingLoadBalancer}. */
+@RunWith(JUnit4.class)
 public class XdsRoutingLoadBalancerTest {
 
   private final List<LoadBalancer> fooBalancers = new ArrayList<>();