[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/syntheticExtensions/plusPlus.kt

@@ -0,0 +1,24 @@
+// DONT_TARGET_EXACT_BACKEND: JS JS_IR JS_IR_ES6 WASM NATIVE
+// MODULE: lib
+// FILE: JavaClass.java
+
+class JavaClass {
+    private int myX = 0;
+
+    public int getX() {
+        return myX;
+    }
+
+    public void setX(int x) {
+        myX = x;
+    }
+}
+
+// MODULE: main(lib)
+// FILE: 1.kt
+
+fun box(): String {
+    val javaClass = JavaClass()
+    javaClass.x++
+    return if (javaClass.x == 1) "OK" else "ERROR"
+}