[FLINK-18368][tests] Cleanup Kerberos settings after test.

That way successive unit tests that reuse the JVM will not suffer from an
incompatible initial configuration.

flink-filesystems/flink-hadoop-fs/src/test/java/org/apache/flink/runtime/util/HadoopUtilsTest.java

@@ -25,6 +25,7 @@
 import org.apache.hadoop.security.UserGroupInformation;
 import org.apache.hadoop.security.UserGroupInformation.AuthenticationMethod;
 import org.apache.hadoop.security.token.Token;
+import org.junit.AfterClass;
 import org.junit.BeforeClass;
 import org.junit.Test;
 import org.mockito.Mockito;
@@ -49,6 +50,11 @@ public static void setPropertiesToEnableKerberosConfigInit() throws KrbException
  sun.security.krb5.Config.refresh();
  }
 
+ @AfterClass
+ public static void cleanupHadoopConfigs() throws KrbException {
+ UserGroupInformation.setConfiguration(new Configuration());
+ }
+
  @Test
  public void testShouldReturnFalseWhenNoKerberosCredentialsOrDelegationTokens() {
  UserGroupInformation.setConfiguration(getHadoopConfigWithAuthMethod(AuthenticationMethod.KERBEROS));