JS: move more test to box tests

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

@@ -0,0 +1,12 @@
+package foo
+
+class myInt(a: Int) {
+    val value = a;
+
+    operator fun plus(other: myInt): myInt = myInt(value + other.value)
+}
+
+fun box(): String {
+
+    return if ((myInt(3) + myInt(5)).value == 8) "OK" else "fail"
+}