JS: move more test to box tests

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

@@ -0,0 +1,19 @@
+package foo
+
+class A(val c: Int) {
+}
+
+
+operator fun A.inc() = A(5)
+operator fun A.dec() = A(10)
+
+fun box(): String {
+    var a = A(1)
+
+    if ((++a).c != 5) return "fail1"
+    if ((a++).c != 5) return "fail2"
+    if ((--a).c != 10) return "fail3"
+    if ((a--).c != 10) return "fail4"
+
+    return "OK"
+}