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-webflux/src/test/kotlin/org/springframework/web/reactive/function/client/ClientResponseExtensionsTests.kt

@@ -21,8 +21,8 @@ import io.mockk.mockk
 import io.mockk.verify
 import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.runBlocking
-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.HttpStatus

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/client/WebClientExtensionsTests.kt

@@ -22,7 +22,7 @@ import io.mockk.verify
 import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.runBlocking
-import org.junit.Assert.assertEquals
+import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Test
 import org.reactivestreams.Publisher
 import org.springframework.core.ParameterizedTypeReference

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/server/CoRouterFunctionDslTests.kt

@@ -16,7 +16,7 @@
 
 package org.springframework.web.reactive.function.server
 
-import org.junit.Assert.fail
+import org.junit.jupiter.api.fail
 import org.junit.jupiter.api.Test
 import org.springframework.core.io.ClassPathResource
 import org.springframework.http.HttpHeaders.*

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/server/RouterFunctionDslTests.kt

@@ -16,7 +16,7 @@
 
 package org.springframework.web.reactive.function.server
 
-import org.junit.Assert.fail
+import org.junit.jupiter.api.fail
 import org.junit.jupiter.api.Test
 import org.springframework.core.io.ClassPathResource
 import org.springframework.http.HttpHeaders.*

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/server/ServerRequestExtensionsTests.kt

@@ -21,8 +21,8 @@ import io.mockk.mockk
 import io.mockk.verify
 import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.runBlocking
-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.codec.multipart.Part

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/server/ServerResponseExtensionsTests.kt

@@ -23,7 +23,7 @@ import io.reactivex.Flowable
 import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.runBlocking
-import org.junit.Assert.assertEquals
+import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Test
 import org.reactivestreams.Publisher
 import org.springframework.core.ParameterizedTypeReference

spring-webflux/src/test/kotlin/org/springframework/web/reactive/result/KotlinInvocableHandlerMethodTests.kt

@@ -21,7 +21,7 @@ import io.mockk.mockk
 import kotlinx.coroutines.delay
 import org.hamcrest.CoreMatchers.`is`
 import org.hamcrest.MatcherAssert.assertThat
-import org.junit.Assert.assertEquals
+import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Test
 import org.springframework.http.HttpStatus
 import org.springframework.http.server.reactive.ServerHttpResponse

spring-webflux/src/test/kotlin/org/springframework/web/reactive/result/method/annotation/CoroutinesIntegrationTests.kt

@@ -23,8 +23,8 @@ import kotlinx.coroutines.async
 import kotlinx.coroutines.delay
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.flow
-import org.junit.Assert.assertEquals
-import org.junit.Assert.fail
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.fail
 import org.springframework.context.ApplicationContext
 import org.springframework.context.annotation.AnnotationConfigApplicationContext
 import org.springframework.context.annotation.ComponentScan