Make sure the utf8 class definition isn't tested

Author: samcv

doc/Type/utf8.pod6

@@ -4,7 +4,8 @@
 
 =SUBTITLE Mutable uint8 buffer for utf8 binary data
 
-    class utf8 does Blob[uint8] { ... }
+=for code :skip-test
+class utf8 does Blob[uint8] { ... }
 
 A C<utf8> is a subtype of Blob which is specificaly uint8 data for holding UTF-8
 encoded text.