CI should run against both GNU and BSD versions of grep, sed, etc.

[mattly]

It's difficult enough keeping this stuff straight myself, and asking contributors to do that is perhaps a bit much. The CI process should run tests with both the GNU and BSD versions of common utilities.

[ricochet1k]

Running on macos might be good too.

[mattly]

Well, macOS uses the BSD utils. Right? Maybe they've got their own variants now that are different, I haven't been keeping up with things.