refactor: PropertiesPropertySourceTest to junit5

Author: Realtin

log4j-api-test/src/test/java/org/apache/logging/log4j/util/PropertiesPropertySourceTest.java

@@ -16,7 +16,7 @@
  */
 package org.apache.logging.log4j.util;
 
-import static org.junit.Assert.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
 
 import java.util.Arrays;
 import java.util.Collections;