KAPT: Run tests on JDK 21

Previously, some tests have also been disabled on JDK 11+, since
we did not filter out empty filesets, which led to failed assertion
inside JDK.
 #KT-57389

plugins/kapt4/build.gradle.kts

@@ -56,6 +56,7 @@ testsJar {}
 kaptTestTask("test", JavaLanguageVersion.of(8))
 kaptTestTask("testJdk11", JavaLanguageVersion.of(11))
 kaptTestTask("testJdk17", JavaLanguageVersion.of(17))
+kaptTestTask("testJdk21", JavaLanguageVersion.of(21))
 
 fun Project.kaptTestTask(name: String, javaLanguageVersion: JavaLanguageVersion) {
     val service = extensions.getByType<JavaToolchainService>()

plugins/kapt4/test/org/jetbrains/kotlin/kapt4/AbstractKotlinKapt4ContextTestBase.kt

@@ -5,6 +5,7 @@
 
 package org.jetbrains.kotlin.kapt4
 
+import org.jetbrains.kotlin.kapt3.base.util.doOpenInternalPackagesIfRequired
 import org.jetbrains.kotlin.kapt3.test.*
 import org.jetbrains.kotlin.kapt3.test.KaptTestDirectives.MAP_DIAGNOSTIC_LOCATIONS
 import org.jetbrains.kotlin.platform.jvm.JvmPlatforms
@@ -22,6 +23,10 @@ open class AbstractKotlinKapt4ContextTest : AbstractKotlinKapt4ContextTestBase(T
 abstract class AbstractKotlinKapt4ContextTestBase(
     targetBackend: TargetBackend
 ) : AbstractKotlinCompilerWithTargetBackendTest(targetBackend) {
+    init {
+        doOpenInternalPackagesIfRequired()
+    }
+
     override fun TestConfigurationBuilder.configuration() {
         globalDefaults {
             frontend = FrontendKinds.FIR