Add logback sample application

Add a logback sample application to demonstrate how `logback-spring.xml`
can be used for customization.

See gh-2558

spring-boot-samples/spring-boot-sample-logback/src/test/java/sample/logback/SampleLogbackApplicationTests.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2012-2014 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 sample.logback;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.springframework.boot.test.OutputCapture;
+
+import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.not;
+
+public class SampleLogbackApplicationTests {
+
+	@Rule
+	public OutputCapture outputCapture = new OutputCapture();
+
+	@Test
+	public void testLoadedCustomLogbackConfig() throws Exception {
+		SampleLogbackApplication.main(new String[0]);
+		this.outputCapture.expect(containsString("Sample Debug Message"));
+		this.outputCapture.expect(not(containsString("Sample Trace Message")));
+	}
+
+}