More control over encoding of test files

The few UTF-8 test files are commented as such, and there is only one
non UTF-8 test file (to test non UTF-8 sources).

testes/utf8.lua

@@ -1,6 +1,8 @@
 -- $Id: testes/utf8.lua $
 -- See Copyright Notice in file all.lua
 
+-- UTF-8 file
+
 print "testing UTF-8 library"
 
 local utf8 = require'utf8'