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>