test_undecodable_code(): set locale to C

The test is still failing on "x86 FreeBSD 7.2 3.x" and "sparc solaris10 gcc
3.x" buildbots. It looks like the locale encoding is able to decode b'\xff'. I
suppose that it is an encoding like 'iso-8859-1'.

Use C locale to set, I hope, the locale encoding to 'ascii'. Display also the
encoding so if the test fails, at least I will learn the locale encoding
choosen for the C locale.