JS: move more test to box tests

js/js.translator/testData/box/operatorOverloading/postfixOnProperty.kt

@@ -0,0 +1,22 @@
+package foo
+
+var a = MyInt()
+
+class MyInt() {
+    var b = 0
+
+    operator fun inc(): MyInt {
+        b = b + 1;
+        return this;
+    }
+}
+
+
+fun box(): String {
+    val d = a++;
+
+    if (a.b != 1) return "fail1: ${a.b}"
+    if (d.b != 1) return "fail2: ${d.b}"
+
+    return "OK"
+}