Add a separate test configurations for JDK 21 dependent test

After it's released, there would be no need in them, but right now
they are unavailable through the toolchain, so we can't require it.
See KT-58765 for tracking

But there should be a dedicated Build configuration with JDK_21_0 env
properly set.

^KT-58716 Fixed

compiler/test-infrastructure-utils/tests/org/jetbrains/kotlin/test/TestJdkKind.kt

@@ -32,6 +32,9 @@ enum class TestJdkKind(val requiresSeparateProcess: Boolean = false) {
     // JDK found at $JDK_17_0
     FULL_JDK_17(requiresSeparateProcess = true),
 
+    // JDK found at $JDK_21_0
+    FULL_JDK_21(requiresSeparateProcess = true),
+
     // JDK found at java.home
     FULL_JDK,
     ANDROID_API,

compiler/test-infrastructure-utils/tests/org/jetbrains/kotlin/test/util/KtTestUtil.java

@@ -159,6 +159,10 @@ public class KtTestUtil {
         return getJdkHome("JDK_17_0", "JDK_17");
     }
 
+    public static File getJdk21Home() {
+        return getJdkHome("JDK_21_0", "JDK_21");
+    }
+
     @NotNull
     public static String getTestDataPathBase() {
         return getHomeDirectory() + "/compiler/testData";