Introduce reset method to OutputCapture

Closes gh-8390

spring-boot-test/src/main/java/org/springframework/boot/test/rule/OutputCapture.java

@@ -91,6 +91,10 @@ public class OutputCapture implements TestRule {
 		this.copy = null;
 	}
 
+	public void reset() {
+		this.copy.reset();
+	}
+
 	public void flush() {
 		try {
 			this.captureOut.flush();

spring-boot-test/src/test/java/org/springframework/boot/test/rule/OutputCaptureTests.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2012-2017 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.springframework.boot.test.rule;
+
+import org.junit.Rule;
+import org.junit.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link OutputCapture}.
+ *
+ * @author Roland Weisleder
+ */
+public class OutputCaptureTests {
+
+	@Rule
+	public OutputCapture outputCapture = new OutputCapture();
+
+	@Test
+	public void toStringShouldReturnAllCapturedOutput() throws Exception {
+		System.out.println("Hello World");
+
+		assertThat(this.outputCapture.toString()).contains("Hello World");
+	}
+
+	@Test
+	public void reset() throws Exception {
+		System.out.println("Hello");
+		this.outputCapture.reset();
+		System.out.println("World");
+
+		assertThat(this.outputCapture.toString()).doesNotContain("Hello")
+				.contains("World");
+	}
+
+}