[hotfix][tests] Add "javax.management.*" to PowerMock ignore list

The test otherwise logged a LinkageError since it tries to load the MBeanServer interface which was already loaded.

flink-metrics/flink-metrics-datadog/src/test/java/org/apache/flink/metrics/datadog/DatadogHttpClientTest.java

@@ -46,7 +46,7 @@
  */
 @RunWith(PowerMockRunner.class)
 @PrepareForTest({DMetric.class, DatadogHttpClient.class})
-@PowerMockIgnore("javax.net.ssl.*")
+@PowerMockIgnore({"javax.net.ssl.*", "javax.management.*"})
 public class DatadogHttpClientTest {
 
  private static List<String> tags = Arrays.asList("tag1", "tag2");