Maker some test classes static.

src/test/java/org/apache/commons/configuration2/TestDefaultImmutableConfiguration.java

@@ -42,7 +42,7 @@
 public class TestDefaultImmutableConfiguration {
 
     /** Tests default methods. This class MUST NOT override the default methods! */
-    private class MapImmutableConfiguration implements ImmutableConfiguration {
+    private static class MapImmutableConfiguration implements ImmutableConfiguration {
 
         Map<String, Object> map = new HashMap<>();
 

src/test/java/org/apache/commons/configuration2/TestDynamicCombinedConfiguration.java

@@ -99,7 +99,7 @@ public void shutdown() {
         }
 
     }
-    private class ReloadThread extends Thread {
+    private static class ReloadThread extends Thread {
         private final CombinedConfigurationBuilder builder;
         private final int[] failures;
         private final int index;

src/test/java/org/apache/commons/configuration2/TestXMLConfiguration.java

@@ -74,7 +74,7 @@ public class TestXMLConfiguration {
     /**
      * A thread used for testing concurrent access to a builder.
      */
-    private class ReloadThread extends Thread {
+    private static class ReloadThread extends Thread {
         private final FileBasedConfigurationBuilder<?> builder;
 
         ReloadThread(final FileBasedConfigurationBuilder<?> confBulder) {

src/test/java/org/apache/commons/configuration2/web/TestServletFilterConfiguration.java

@@ -33,7 +33,7 @@
  *
  */
 public class TestServletFilterConfiguration extends TestAbstractConfiguration {
-    private class MockFilterConfig implements FilterConfig {
+    private static class MockFilterConfig implements FilterConfig {
         private final Properties parameters = new Properties();
 
         @Override