util: Add RunWith to RandomSubsettingLaodBalancerTest

In some environments the test won't run without the annotation.

Author: ejona86

util/src/test/java/io/grpc/util/RandomSubsettingLoadBalancerTest.java

@@ -47,6 +47,8 @@
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
 import org.mockito.ArgumentCaptor;
 import org.mockito.Captor;
 import org.mockito.Mock;
@@ -55,6 +57,7 @@
 import org.mockito.junit.MockitoRule;
 import org.mockito.stubbing.Answer;
 
+@RunWith(JUnit4.class)
 public class RandomSubsettingLoadBalancerTest {
   @Rule
   public final MockitoRule mockitoRule = MockitoJUnit.rule();