JS: move more test to box tests

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

@@ -0,0 +1,18 @@
+package foo
+
+class MyInt() {
+    var b = 0
+
+    operator fun dec(): MyInt {
+        val res = MyInt()
+        res.b++;
+        return res;
+    }
+}
+
+
+fun box(): String {
+    var c = MyInt()
+    --c;
+    return if (c.b == 1) "OK" else "fail: ${c.b}"
+}