refactor(SourceTest): use junit5

Author: hulkoba

log4j-core-test/src/test/java/org/apache/logging/log4j/core/util/SourceTest.java

@@ -16,7 +16,7 @@
  */
 package org.apache.logging.log4j.core.util;
 
-import static org.junit.Assert.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
 
 import java.io.File;
 import java.net.MalformedURLException;