Provide DelegatingAppender class to allow SLF4J logging provider Appenders to be injected [programmatically] at configuration/runtime.

This is very useful in Integration Tests where the test class needs to assert log statements in infrastructure, system, framework or application code.

Resolves gh-73.

spring-geode-starter-logging/src/test/java/org/springframework/geode/logging/slf4j/logback/DelegatingAppenderUnitTests.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2019 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
+ *
+ *     https://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.springframework.geode.logging.slf4j.logback;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+
+import org.junit.Test;
+
+import ch.qos.logback.core.Appender;
+import ch.qos.logback.core.helpers.NOPAppender;
+
+/**
+ * Unit Tests for {@link DelegatingAppender}.
+ *
+ * @author John Blum
+ * @see org.junit.Test
+ * @see org.mockito.Mock
+ * @see org.mockito.Mockito
+ * @see org.springframework.geode.logging.slf4j.logback.DelegatingAppender
+ * @since 1.3.0
+ */
+public class DelegatingAppenderUnitTests {
+
+	@Test
+	public void delegatingAppenderDefaultsToNoOpAppender() {
+		assertThat(new DelegatingAppender<>().getAppender()).isInstanceOf(NOPAppender.class);
+	}
+
+	@Test
+	@SuppressWarnings({ "rawtypes", "unchecked" })
+	public void delegatingAppenderDelegatesToMockAppender() {
+
+		Appender mockAppender = mock(Appender.class);
+
+		DelegatingAppender delegatingAppender = new DelegatingAppender<>();
+
+		delegatingAppender.setAppender(mockAppender);
+
+		assertThat(delegatingAppender.getAppender()).isSameAs(mockAppender);
+
+		delegatingAppender.append("TEST");
+
+		verify(mockAppender, times(1)).doAppend(eq("TEST"));
+	}
+}