Add substr tests which will throw UTF-16 implementations which are ac…

…ting like UCS-2.

S32-str/substr.t

@@ -2,7 +2,7 @@ use v6;
 
 use Test;
 
-plan 87;
+plan 90;
 
 # L<S32::Str/Str/=item substr>
 
@@ -64,6 +64,13 @@ plan 87;
     is("שיעבוד כבר".substr(4, 4), "וד כ", ".substr on Hebrew text");
 }
 
+{ # codepoints greater than 0xFFFF
+    my $str = (0x10426, 0x10427).chrs;
+    is $str.codes, 2, "Sanity check string";
+    is substr($str, 0, 1), 0x10426.chr, "Taking first char of Deseret string";
+    is substr($str, 1, 1), 0x10427.chr, "Taking second char of Deseret string";
+}
+
 sub l (Int $a) {  my $l = $a; return $l }
 
 #Substr with StrLen