add haxe.Utf8.sub tests

tests/unit/unitstd/haxe/Utf8.unit.hx

@@ -1,5 +1,4 @@
-/*
- // disable until we decide how to handle JS/SWF API being UCS2 and not UTF8
+#if false
 var str = "あ𠀀い";
 haxe.Utf8.length(str) == 3;
 haxe.Utf8.charCodeAt(str, 0) == 0x3042;
@@ -10,4 +9,10 @@ buf.addChar(0x3042);
 buf.addChar(0x20000);
 buf.addChar(0x3044);
 buf.toString() == str;
-*/
+haxe.Utf8.compare(haxe.Utf8.sub(str, 0, 3), str) == 0;
+haxe.Utf8.compare(haxe.Utf8.sub(str, 0, 2), "あ𠀀") == 0;
+haxe.Utf8.compare(haxe.Utf8.sub(str, 1, 2), "𠀀い") == 0;
+haxe.Utf8.compare(haxe.Utf8.sub(str, 0, 0), "") == 0;
+haxe.Utf8.compare(haxe.Utf8.sub(str, 1, 0), "") == 0;
+haxe.Utf8.compare(haxe.Utf8.sub(str, 9, 0), "") == 0;
+#end