Introduce Unit Tests

First off; thanks for this amazing port from OpenBSD.

For a long while I wanted to contribute something and understand vi's internals, however my C is a bit rusty (taught a college level course years ago and worked on some projects) so it was always pretty intimidating to get into this code base.

Currently, AI is helping me to get a grasp of what is going on here and I realized that tests on vi's behavior are missing.

Now, I forked the repo and played around with a coding assistant (Claude) and it generated a pretty nice initial test harness for this code repo that is purely written in POSIX sh.

I would like to open a PR but first I wanted to know: is this even something you would consider? I would surely test and proof-read the provided POSIX sh solution and integrate it with Github Actions, so a lot of the architectures listed in your README (everything except AIX, Solaris, illumos, Windows/Cygwin as they are not available as Github Action runners, not even as a VM) would be covered.

However, if you absolutely don't want any AI generated solutions or if you're not interested in a test harness I wont bother you with it!

Thanks again!