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-test/src/test/java/org/springframework/boot/test/system/OutputCaptureRuleTests.java

@@ -29,12 +29,18 @@ import static org.assertj.core.api.Assertions.assertThat;
 public class OutputCaptureRuleTests {
 
 	@Rule
-	public OutputCaptureRule outputCapture = new OutputCaptureRule();
+	public OutputCaptureRule output = new OutputCaptureRule();
 
 	@Test
 	public void toStringShouldReturnAllCapturedOutput() {
 		System.out.println("Hello World");
-		assertThat(this.outputCapture.toString()).contains("Hello World");
+		assertThat(this.output.toString()).contains("Hello World");
+	}
+
+	@Test
+	public void captureShouldBeAssertable() {
+		System.out.println("Hello World");
+		assertThat(this.output).contains("Hello World");
 	}
 
 }