Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

compiler/testData/codegen/box/objects/kt2398.kt

@@ -0,0 +1,18 @@
+class C {
+    public object Obj {
+        val o = "O"
+
+        object InnerObj {
+          fun k() = "K"
+        }
+
+        class D {
+            val ko = "KO"
+        }
+    }
+}
+
+fun box(): String {
+    val res = C().Obj.o + C().Obj.InnerObj.k()  + C().Obj.D().ko
+    return if (res == "OKKO") "OK" else "Fail: $res"
+}