Return Logback to its default basic configuration after clean up

See gh-4026

Author: wilkinsona

spring-boot/src/main/java/org/springframework/boot/logging/logback/LogbackLoggingSystem.java

@@ -37,6 +37,7 @@
 import org.springframework.util.ResourceUtils;
 import org.springframework.util.StringUtils;
 
+import ch.qos.logback.classic.BasicConfigurator;
 import ch.qos.logback.classic.Level;
 import ch.qos.logback.classic.LoggerContext;
 import ch.qos.logback.classic.joran.JoranConfigurator;
@@ -178,6 +179,7 @@ public void cleanUp() {
 		super.cleanUp();
 		getLoggerContext().getStatusManager().clear();
 		getLoggerContext().stop();
+		BasicConfigurator.configure(getLoggerContext());
 	}
 
 	@Override