JS: move more test to box tests

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

@@ -0,0 +1,17 @@
+package foo
+
+class A() {
+
+    var message = ""
+    operator fun plusAssign(other: A) {
+        message = message + "!"
+    }
+
+}
+
+fun box(): String {
+    var c = A()
+    c += A()
+    c += A()
+    return if (c.message == "!!") return "OK" else "fail"
+}