[Test] Properly merge box against java testdata into codegen/box

Previous commit about it was 3199ce03 and it was completely wrong

compiler/testData/codegen/box/reflection/mapping/jClass2kClass.kt

@@ -0,0 +1,19 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+// DONT_TARGET_EXACT_BACKEND: JS JS_IR JS_IR_ES6 WASM NATIVE
+// WITH_RUNTIME
+// MODULE: lib
+// FILE: J.java
+
+public class J {}
+
+// MODULE: main(lib)
+// FILE: 1.kt
+
+import kotlin.test.assertEquals
+
+fun box(): String {
+    val j = J::class.java
+    assertEquals(j, j.kotlin.java)
+
+    return "OK"
+}