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-webmvc/src/test/java/org/springframework/web/servlet/view/ResourceBundleViewResolverTests.java

@@ -34,7 +34,7 @@ import org.springframework.web.servlet.View;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
-import static org.junit.Assume.assumeTrue;
+import static org.junit.jupiter.api.Assumptions.assumeTrue;
 
 /**
  * @author Rod Johnson

spring-webmvc/src/test/kotlin/org/springframework/web/servlet/function/RouterFunctionDslTests.kt

@@ -16,9 +16,9 @@
 
 package org.springframework.web.servlet.function
 
-import org.junit.Assert.assertFalse
-import org.junit.Assert.assertTrue
-import org.junit.Assert.fail
+import org.junit.jupiter.api.Assertions.assertFalse
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.fail
 import org.junit.jupiter.api.Test
 import org.springframework.core.io.ClassPathResource
 import org.springframework.http.HttpHeaders.*

spring-webmvc/src/test/kotlin/org/springframework/web/servlet/function/ServerRequestExtensionsTests.kt

@@ -19,8 +19,8 @@ package org.springframework.web.servlet.function
 import io.mockk.every
 import io.mockk.mockk
 import io.mockk.verify
-import org.junit.Assert.assertEquals
-import org.junit.Assert.assertNull
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertNull
 import org.junit.jupiter.api.Test
 import org.springframework.core.ParameterizedTypeReference
 import org.springframework.http.MediaType

spring-webmvc/src/test/kotlin/org/springframework/web/servlet/function/ServerResponseExtensionsTests.kt

@@ -19,10 +19,10 @@ package org.springframework.web.servlet.function
 import io.mockk.every
 import io.mockk.mockk
 import io.mockk.verify
-import org.junit.Assert
 import org.junit.jupiter.api.Test
 import org.springframework.core.ParameterizedTypeReference
 import java.util.*
+import org.junit.jupiter.api.Assertions.assertEquals
 
 /**
  * Tests for WebMvc.fn [ServerResponse] extensions.
@@ -38,7 +38,7 @@ class ServerResponseExtensionsTests {
 		val body = Arrays.asList("foo", "bar")
 		val typeReference = object: ParameterizedTypeReference<List<String>>() {}
 		every { builder.body(body, typeReference) } returns response
-		Assert.assertEquals(response, builder.bodyWithType<List<String>>(body))
+		assertEquals(response, builder.bodyWithType<List<String>>(body))
 		verify { builder.body(body, typeReference) }
 	}
 }

spring-webmvc/src/test/kotlin/org/springframework/web/servlet/mvc/method/annotation/ServletAnnotationControllerHandlerMethodKotlinTests.kt

@@ -16,7 +16,7 @@
 
 package org.springframework.web.servlet.mvc.method.annotation
 
-import org.junit.Assert.*
+import org.junit.jupiter.api.Assertions.*
 import org.junit.jupiter.api.Test
 import org.springframework.mock.web.test.MockHttpServletRequest
 import org.springframework.mock.web.test.MockHttpServletResponse