Ignore existing output when asserting captured log output

Closes gh-12734

spring-boot/src/test/java/org/springframework/boot/logging/LoggingApplicationListenerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -201,8 +201,10 @@ public class LoggingApplicationListenerTests {
 		this.initializer.initialize(this.context.getEnvironment(),
 				this.context.getClassLoader());
 		Log logger = LogFactory.getLog(LoggingApplicationListenerTests.class);
+		String existingOutput = this.outputCapture.toString();
 		logger.info("Hello world");
-		String output = this.outputCapture.toString().trim();
+		String output = this.outputCapture.toString().substring(existingOutput.length())
+				.trim();
 		assertThat(output).startsWith("target/foo.log");
 	}
 
@@ -226,8 +228,10 @@ public class LoggingApplicationListenerTests {
 		this.initializer.initialize(this.context.getEnvironment(),
 				this.context.getClassLoader());
 		Log logger = LogFactory.getLog(LoggingApplicationListenerTests.class);
+		String existingOutput = this.outputCapture.toString();
 		logger.info("Hello world");
-		String output = this.outputCapture.toString().trim();
+		String output = this.outputCapture.toString().substring(existingOutput.length())
+				.trim();
 		assertThat(output).startsWith("target/foo/spring.log");
 	}