[Test] Replace public fun box() with fun box() in all box tests

compiler/testData/codegen/box/increment/prefixIncrementOnClassSmartCast.kt

@@ -2,7 +2,7 @@ open class Base
 class Derived: Base()
 operator fun Derived.inc(): Derived { return Derived() }
 
-public fun box() : String {
+fun box() : String {
     var i : Base
     i = Derived()
     val j = ++i