Converting tests to junit5 (#4699)

* started converting tests to junit5

* replaced TempFolder with JUnit TempDir

* migrated some test classes to JUnit5

* migrated a parameterized testclass to junit5

* Update core/pom.xml

Co-authored-by: Mark Waite <mark.earl.waite@gmail.com>

* replaced junit4 with junit5-vintage engine

Co-authored-by: Mark Waite <mark.earl.waite@gmail.com>

core/src/test/java/hudson/ProxyConfigurationTest.java

@@ -23,12 +23,12 @@
  */
 package hudson;
 
-import static org.junit.Assert.assertEquals;
-
-import org.junit.Test;
+import org.junit.jupiter.api.Test;
 
 import java.net.Proxy;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
 public class ProxyConfigurationTest {
 
     @Test