Migrate tests to JUnit5 (#142)

* Migrate annotations and imports
* Migrate assertions
* Remove public visibility for test classes and methods
* Minor code cleanup

Author: strangelookingnerd

src/test/java/hudson/plugins/build_timeout/BuildStepWithTimeoutTest.java

@@ -20,11 +20,11 @@
 @WithJenkins
 class BuildStepWithTimeoutTest {
 
-    private final static long TINY_DELAY = 100L;
-    private final static long HUGE_DELAY = 5000L;
+    private static final long TINY_DELAY = 100L;
+    private static final long HUGE_DELAY = 5000L;
 
     @BeforeEach
-    public void before() {
+    void before() {
         // this allows timeout shorter than 3 minutes.
         BuildTimeoutWrapper.MINIMUM_TIMEOUT_MILLISECONDS = 100;
     }