[PATCH/RFC] tests/: Add t0111-filter.sh

[Jason A. Donenfeld]

Terrific! I was hoping something like this would come about. Looking
forward to seeing the lua side of things.

There's another change I'm going to be pushing later today that
involves being able to do meaningful things with the
cgit_filter_close() exit code. I'll then be using this to /maybe/
implement a basic authentication infrastructure. Stand by! Exciting
things are happening.