MINOR: Add missing @Test annotation to MetadataTest#testMetadataMerge (#8141)

Reviewers: Brian Byrne <bbyrne@confluent.io>, Rajini Sivaram <rajinisivaram@googlemail.com>

clients/src/test/java/org/apache/kafka/clients/MetadataTest.java

@@ -762,6 +762,7 @@ public class MetadataTest {
         return brokers;
     }
 
+    @Test
     public void testMetadataMerge() {
         Time time = new MockTime();