[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/overrideOnlyGetter.kt

@@ -0,0 +1,23 @@
+// DONT_TARGET_EXACT_BACKEND: JS JS_IR JS_IR_ES6 WASM NATIVE
+// MODULE: lib
+// FILE: JavaClass2.java
+
+class JavaClass1 {
+    protected Object value = null;
+
+    public Object getSomething() { return null; }
+    public void setSomething(Object value) {  this.value = value; }
+}
+
+class JavaClass2 extends JavaClass1 {
+    public String getSomething() { return (String)value; }
+}
+
+// MODULE: main(lib)
+// FILE: 1.kt
+
+fun box(): String {
+    val javaClass = JavaClass2()
+    javaClass.something = "OK"
+    return javaClass.something
+}