[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/setterNonVoid1.kt

@@ -0,0 +1,27 @@
+// DONT_TARGET_EXACT_BACKEND: JS JS_IR JS_IR_ES6 WASM NATIVE
+// MODULE: lib
+// FILE: JavaClass.java
+
+class JavaClass {
+    private String myX = "";
+
+    public String getX() {
+        return myX;
+    }
+
+    public JavaClass setX(String x) {
+        myX = x;
+        return this;
+    }
+}
+
+// MODULE: main(lib)
+// FILE: 1.kt
+
+fun box(): String {
+    val javaClass = JavaClass()
+    if (javaClass.x == "") {
+        javaClass.x = "OK"
+    }
+    return javaClass.x
+}