JS: move more test to box tests

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

@@ -0,0 +1,24 @@
+package foo
+
+import java.util.*
+
+operator fun <T> ArrayList<T>.plus(other: Collection<T>): List<T> {
+    val c = ArrayList<T>()
+    c.addAll(this)
+    c.addAll(other)
+    return c
+}
+
+fun box(): String {
+    var v1 = ArrayList<String>()
+    v1.add("foo")
+    val v2 = ArrayList<String>()
+    v2.add("bar")
+    val v = v1 + v2
+
+    if (v.size != 2) return "fail1: ${v.size}"
+    if (v[0] != "foo") return "fail2: ${v[0]}"
+    if (v[1] != "bar") return "fail3: ${v[1]}"
+
+    return "OK"
+}