[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/javaConstructor.kt

@@ -0,0 +1,28 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+// DONT_TARGET_EXACT_BACKEND: JS JS_IR JS_IR_ES6 WASM NATIVE
+// WITH_REFLECT
+// MODULE: lib
+// FILE: J.java
+
+public class J {
+    public final String result;
+
+    public J(String result) {
+        this.result = result;
+    }
+}
+
+// MODULE: main(lib)
+// FILE: 1.kt
+
+import kotlin.reflect.*
+import kotlin.reflect.jvm.*
+
+fun box(): String {
+    val reference = ::J
+    val javaConstructor = reference.javaConstructor ?: return "Fail: no Constructor for reference"
+    val j = javaConstructor.newInstance("OK")
+    val kotlinConstructor = javaConstructor.kotlinFunction
+    if (reference != kotlinConstructor) return "Fail: reference != kotlinConstructor"
+    return j.result
+}