Allow OutputCaptureRule to be used with AssertJ

Update `OutputCaptureRule` so that it now implements
`CapturedOutput` and can be used directly with AssertJ.

Closes gh-17512

spring-boot-project/spring-boot-tools/spring-boot-test-support/src/main/java/org/springframework/boot/testsupport/system/OutputCaptureRule.java

@@ -34,7 +34,7 @@ import static org.hamcrest.Matchers.allOf;
  * @author Andy Wilkinson
  * @since 2.2.0
  */
-public class OutputCaptureRule implements TestRule {
+public class OutputCaptureRule implements TestRule, CapturedOutput {
 
 	private final OutputCapture delegate = new OutputCapture();
 
@@ -64,6 +64,21 @@ public class OutputCaptureRule implements TestRule {
 		};
 	}
 
+	@Override
+	public String getAll() {
+		return this.delegate.getAll();
+	}
+
+	@Override
+	public String getOut() {
+		return this.delegate.getOut();
+	}
+
+	@Override
+	public String getErr() {
+		return this.delegate.getErr();
+	}
+
 	@Override
 	public String toString() {
 		return this.delegate.toString();