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-extensions/src/testFixtures/java/org/jivesoftware/util/Verification.java

@@ -0,0 +1,105 @@
+/**
+ *
+ * Copyright the original author or authors
+ *
+ * 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.util;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.jivesoftware.smack.packet.IQ;
+import org.jivesoftware.smack.packet.Stanza;
+
+/**
+ * Implement this interface to verify a request/response pair.
+ * <p>
+ * For convenience there are some useful predefined implementations.
+ *
+ * @param <T> class of the request
+ * @param <S> class of the response
+ *
+ * @author Henning Staib
+ */
+public interface Verification<T extends Stanza, S extends Stanza> {
+
+    /**
+     * Verifies that the "To" field of the request corresponds with the "From" field of
+     * the response.
+     */
+    Verification<Stanza, Stanza> correspondingSenderReceiver = new Verification<Stanza, Stanza>() {
+
+        @Override
+        public void verify(Stanza request, Stanza response) {
+            assertEquals(response.getFrom(), request.getTo());
+        }
+
+    };
+
+    /**
+     * Verifies that the type of the request is a GET.
+     */
+    Verification<IQ, Stanza> requestTypeGET = new Verification<IQ, Stanza>() {
+
+        @Override
+        public void verify(IQ request, Stanza response) {
+            assertEquals(IQ.Type.get, request.getType());
+        }
+
+    };
+
+    /**
+     * Verifies that the type of the request is a SET.
+     */
+    Verification<IQ, Stanza> requestTypeSET = new Verification<IQ, Stanza>() {
+
+        @Override
+        public void verify(IQ request, Stanza response) {
+            assertEquals(IQ.Type.set, request.getType());
+        }
+
+    };
+
+    /**
+     * Verifies that the type of the request is a RESULT.
+     */
+    Verification<IQ, Stanza> requestTypeRESULT = new Verification<IQ, Stanza>() {
+
+        @Override
+        public void verify(IQ request, Stanza response) {
+            assertEquals(IQ.Type.result, request.getType());
+        }
+
+    };
+
+    /**
+     * Verifies that the type of the request is an ERROR.
+     */
+    Verification<IQ, Stanza> requestTypeERROR = new Verification<IQ, Stanza>() {
+
+        @Override
+        public void verify(IQ request, Stanza response) {
+            assertEquals(IQ.Type.error, request.getType());
+        }
+
+    };
+
+    /**
+     * Implement this method to make assertions of the request/response pairs.
+     *
+     * @param request the request collected by the mocked XMPP connection
+     * @param response the response added to the protocol instance
+     */
+    void verify(T request, S response);
+
+}