[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/property/fieldAccessFromExtensionInTraitImpl.kt

@@ -0,0 +1,22 @@
+// DONT_TARGET_EXACT_BACKEND: JS JS_IR JS_IR_ES6 WASM NATIVE
+// MODULE: lib
+// FILE: D.java
+
+public class D {
+    public final String result = "OK";
+}
+
+// MODULE: main(lib)
+// FILE: 1.kt
+
+// KT-4878
+
+interface T {
+    fun Int.foo(d: D) = d.result!!
+}
+
+class A : T {
+    fun bar() = 42.foo(D())
+}
+
+fun box() = A().bar()