JS backend : added test for local extension function.

js/js.translator/testData/expression/function/cases/localExtFunction.kt

@@ -0,0 +1,9 @@
+package foo
+
+fun box(): String {
+    fun String.test(i: Int) = this + i + "OK"
+    val a = "foo".test(32)
+    if (a != "foo32OK") return "$a"
+
+    return "OK"
+}