Introduce test fixtures

This also removes the powermock dependency. Although powermock is a
fine library, it currently prevents dropping Junit4. And since we only
use the Whitebox API of powermock, this simply replaced powermock's
Whitebox with our own.

smack-core/src/testFixtures/java/org/jivesoftware/smack/DummyConnection.java

@@ -14,7 +14,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-
 package org.jivesoftware.smack;
 
 import java.io.IOException;

smack-core/src/testFixtures/java/org/jivesoftware/smack/test/util/CharSequenceEquals.java

@@ -1,6 +1,6 @@
 /**
  *
- * Copyright © 2014 Florian Schmaus
+ * Copyright © 2014-2020 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,7 +17,6 @@
 package org.jivesoftware.smack.test.util;
 
 import org.hamcrest.Description;
-import org.hamcrest.Factory;
 import org.hamcrest.Matcher;
 import org.hamcrest.TypeSafeMatcher;
 
@@ -40,7 +39,6 @@ public class CharSequenceEquals extends TypeSafeMatcher<CharSequence> {
         return charSequenceString.equals(itemString);
     }
 
-    @Factory
     public static Matcher<CharSequence> equalsCharSequence(CharSequence charSequence) {
         return new CharSequenceEquals(charSequence);
     }

smack-core/src/testFixtures/java/org/jivesoftware/smack/test/util/MemoryLeakTestUtil.java

@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2019 Florian Schmaus
+ * Copyright 2019-2020 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -14,9 +14,9 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-package org.jivesoftware.smack.util;
+package org.jivesoftware.smack.test.util;
 
-import static org.junit.Assert.assertNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.fail;
 
 import java.lang.ref.PhantomReference;
@@ -129,7 +129,7 @@ public class MemoryLeakTestUtil {
         }
 
         Reference<?> reference = referenceQueue.poll();
-        assertNull("Reference queue is not empty when it should be", reference);
+        assertNull(reference, "Reference queue is not empty when it should be");
     }
 
     private static void assertReferencesQueueIsEmpty(ReferenceQueue<?> referenceQueue) {

smack-core/src/testFixtures/java/org/jivesoftware/smack/test/util/NetworkUtil.java

@@ -14,7 +14,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-package org.jivesoftware.smack.util;
+package org.jivesoftware.smack.test.util;
 
 import java.io.IOException;
 import java.net.BindException;

smack-core/src/testFixtures/java/org/jivesoftware/smack/test/util/Whitebox.java

@@ -0,0 +1,44 @@
+/**
+ *
+ * Copyright 2020 Florian Schmaus
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.jivesoftware.smack.test.util;
+
+import java.lang.reflect.Field;
+
+import org.apache.commons.lang3.reflect.FieldUtils;
+
+public class Whitebox {
+
+    public static <T> T getInternalState(Object object, String fieldName, Class<T> fieldType) {
+        Class<?> objectClass = object.getClass();
+
+        Field field = FieldUtils.getField(objectClass, fieldName, true);
+
+        Object res;
+        try {
+            res = field.get(object);
+        } catch (IllegalArgumentException | IllegalAccessException e) {
+            throw new AssertionError(e);
+        }
+
+        if (!fieldType.isInstance(res)) {
+            throw new AssertionError(res + " is not a (sub)type of " + fieldType);
+        }
+
+        return fieldType.cast(res);
+    }
+
+}