MINOR: Fixed some resource leaks. (#8922)

Reviewers: Manikumar Reddy <manikumar.reddy@gmail.com>

clients/src/main/java/org/apache/kafka/clients/producer/MockProducer.java

@@ -69,7 +69,7 @@ public class MockProducer<K, V> implements Producer<K, V> {
     private boolean producerFenced;
     private boolean sentOffsets;
     private long commitCount = 0L;
-    private Map<MetricName, Metric> mockMetrics;
+    private final Map<MetricName, Metric> mockMetrics;
 
     public RuntimeException initTransactionException = null;
     public RuntimeException beginTransactionException = null;

clients/src/test/java/org/apache/kafka/common/memory/GarbageCollectedMemoryPoolTest.java

@@ -80,6 +80,7 @@ public class GarbageCollectedMemoryPoolTest {
         GarbageCollectedMemoryPool pool = new GarbageCollectedMemoryPool(1000, 10, true, null);
         ByteBuffer fellOffATruck = ByteBuffer.allocate(1);
         pool.release(fellOffATruck);
+        pool.close();
     }
 
     @Test

clients/src/test/java/org/apache/kafka/common/network/SslTransportLayerTest.java

@@ -903,6 +903,7 @@ public class SslTransportLayerTest {
             NetworkTestUtils.waitForChannelClose(selector, node, ChannelState.State.AUTHENTICATION_FAILED);
             server.close();
             selector.close();
+            serverChannelBuilder.close();
         }
     }
 

clients/src/test/java/org/apache/kafka/common/security/oauthbearer/internals/expiring/ExpiringCredentialRefreshingLoginTest.java

@@ -368,6 +368,7 @@ public class ExpiringCredentialRefreshingLoginTest {
                         inOrder.verify(mockLoginContext).login();
                     }
                 }
+                testExpiringCredentialRefreshingLogin.close();
             }
         }
     }