t0051: add UTF-8 test file

This new file comes from:

https://www.cl.cam.ac.uk/~mgk25/ucs/examples/UTF-8-test.txt

It is licensed under CC BY 4.0.

After discussing that with its author, I think it is ok to
add it as is to our repository. The only restriction might
be that we should indicate in the file any change we make
to it.

License: CC BY 4.0
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>