Add more codec tests (from Cyphre)

Originally proposed by Richard Smolak in
https://gist.github.com/cyphre/9254624

To not have fixture data modified by Git, add an exception to
gitattributes.
Improve .gitattributes config & comment to be explicit about the effects

NB: removing the "test" flag as it is misleading. It is basically
meaningless with eol=lf. In this combination, `text` does _not_ lead to
auto-detection of text files.