Increase the level of JUL logging calls in the tests to try to fix CI

The CI build is failing, but, for some reason, the same tests are
working fine on developer machines. This commit increases the level
at which the logging calls are being made to severe in the hope that
some CI-specific configuration is filtering out the calls when
they're at info level.

Author: wilkinsona

spring-boot/src/test/java/org/springframework/boot/logging/log4j/Log4JLoggingSystemTests.java

@@ -95,7 +95,7 @@ public void loggingThatUsesJulIsCaptured() {
 		this.loggingSystem.initialize();
 		java.util.logging.Logger julLogger = java.util.logging.Logger
 				.getLogger(getClass().getName());
-		julLogger.info("Hello world");
+		julLogger.severe("Hello world");
 		String output = this.output.toString().trim();
 		assertTrue("Wrong output:\n" + output, output.contains("Hello world"));
 	}

spring-boot/src/test/java/org/springframework/boot/logging/log4j2/Log4J2LoggingSystemTests.java

@@ -96,7 +96,7 @@ public void loggingThatUsesJulIsCaptured() {
 		this.loggingSystem.initialize();
 		java.util.logging.Logger julLogger = java.util.logging.Logger
 				.getLogger(getClass().getName());
-		julLogger.info("Hello world");
+		julLogger.severe("Hello world");
 		String output = this.output.toString().trim();
 		assertTrue("Wrong output:\n" + output, output.contains("Hello world"));
 	}