JS: move more test to box tests

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

@@ -0,0 +1,16 @@
+package foo
+
+open class Foo<out T>(open val value: T)
+open class MutableFoo<T>(override var value: T): Foo<T>(value)
+
+operator fun <T> Foo<T>.plus(x: T): Foo<T> = Foo(x)
+
+// overloading:
+operator fun <T> MutableFoo<T>.plus(x: T): MutableFoo<T> = MutableFoo(x)
+
+
+fun box(): String {
+    var f = MutableFoo(1)
+    f += 2
+    return if (f is MutableFoo && f.value == 2) "OK" else "fail"
+}