JS backend: introduce constants for default test package(foo) and default test function(box).

js/js.translator/testFiles/propertyAccess/cases/setter.kt

@@ -4,7 +4,7 @@ class Test() {
     var a : Int = 1
 }
 
-fun f() : Int {
+fun box() : Int {
     var a = Test()
     var b = Test()
     b.a = 100

js/js.translator/testFiles/stdlib/cases/browserDocumentAccess.kt

@@ -3,7 +3,7 @@ package test.browser
 import js.dom.html.document
 import org.w3c.dom.Node
 
-fun foo(): String {
+fun box(): String {
     val element = document.getElementById("foo")!!
     val textNode = document.createTextNode("Some Dynamically Created Content!!!")
     element.appendChild(textNode)