JS: move more test to box tests

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

@@ -0,0 +1,15 @@
+package foo
+
+class MyInt(i: Int) {
+    var b = i
+    operator fun inc(): MyInt {
+        b++;
+        return this;
+    }
+}
+
+fun box(): String {
+    var t = MyInt(0)
+    t++;
+    return if (t.b == 1) "OK" else "fail: ${t.b}"
+}