context: add RunWith annotation

diff --git a/context/src/test/java/io/grpc/PersistentHashArrayMappedTrieTest.java b/context/src/test/java/io/grpc/PersistentHashArrayMappedTrieTest.java
index 6c773f3..19bc1ef 100644
--- a/context/src/test/java/io/grpc/PersistentHashArrayMappedTrieTest.java
+++ b/context/src/test/java/io/grpc/PersistentHashArrayMappedTrieTest.java
@@ -25,7 +25,10 @@
 import io.grpc.PersistentHashArrayMappedTrie.Leaf;
 import io.grpc.PersistentHashArrayMappedTrie.Node;
 import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
 
+@RunWith(JUnit4.class)
 public class PersistentHashArrayMappedTrieTest {
   @Test
   public void leaf_replace() {