Remove JUnit 4 dependency from all modules except spring-test

This commit removes the JUnit 4 dependency from all modules except
spring-test which provides explicit JUnit 4 support.

This commit also includes the following.

- migration from JUnit 4 assertions to JUnit Jupiter assertions in all
  Kotlin tests
- migration from JUnit 4 assumptions in Spring's TestGroup support to
  JUnit Jupiter assumptions, based on org.opentest4j.TestAbortedException
- introduction of a new TestGroups utility class than can be used from
  existing JUnit 4 tests in the spring-test module in order to perform
  assumptions using JUnit 4's Assume class

See gh-23451

spring-test/src/test/java/org/springframework/test/context/junit4/concurrency/SpringJUnit4ConcurrencyTests.java

@@ -40,10 +40,11 @@ import org.springframework.test.context.web.socket.WebSocketServletServerContain
 import org.springframework.test.web.client.samples.SampleTests;
 import org.springframework.test.web.servlet.samples.context.JavaConfigTests;
 import org.springframework.test.web.servlet.samples.context.WebAppResourceTests;
-import org.springframework.tests.Assume;
 import org.springframework.tests.TestGroup;
+import org.springframework.tests.TestGroups;
 import org.springframework.util.ReflectionUtils;
 
+import static org.junit.Assume.assumeTrue;
 import static org.springframework.core.annotation.AnnotatedElementUtils.hasAnnotation;
 import static org.springframework.test.context.junit4.JUnitTestingUtils.runTestsAndAssertCounters;
 
@@ -97,7 +98,8 @@ public class SpringJUnit4ConcurrencyTests {
 
 	@BeforeClass
 	public static void abortIfLongRunningTestGroupIsNotEnabled() {
-		Assume.group(TestGroup.LONG_RUNNING);
+		assumeTrue("TestGroup " + TestGroup.LONG_RUNNING + " is not active.",
+			TestGroups.isGroupActive(TestGroup.LONG_RUNNING));
 	}
 
 	@Test

spring-test/src/test/java/org/springframework/test/web/servlet/htmlunit/DelegatingWebConnectionTests.java

@@ -36,10 +36,11 @@ import org.springframework.stereotype.Controller;
 import org.springframework.test.web.servlet.MockMvc;
 import org.springframework.test.web.servlet.htmlunit.DelegatingWebConnection.DelegateWebConnection;
 import org.springframework.test.web.servlet.setup.MockMvcBuilders;
-import org.springframework.tests.Assume;
 import org.springframework.tests.TestGroup;
+import org.springframework.tests.TestGroups;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.Assume.assumeTrue;
 import static org.mockito.BDDMockito.given;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.verifyNoMoreInteractions;
@@ -125,7 +126,8 @@ public class DelegatingWebConnectionTests {
 
 	@Test
 	public void verifyExampleInClassLevelJavadoc() throws Exception {
-		Assume.group(TestGroup.PERFORMANCE);
+		assumeTrue("TestGroup " + TestGroup.PERFORMANCE + " is not active.",
+			TestGroups.isGroupActive(TestGroup.PERFORMANCE));
 
 		WebClient webClient = new WebClient();