Run ./contrib/test.sh on OSX (and Windows?) in github actions

[nothingmuch]

For context, I introduced a dependency on flock which it turns out is not a standard POSIX shell command (#878), thereby breaking the test script on Dan's machine

To ensure tooling continues to work for all contributors, we should consider running at least a
minimal selection of tests on non-Linux platforms.

[spacebear21]

As a macOS user myself, I'm not convinced that this is actually worth adding to CI. The macOS runners are slower and less reliable than the ubuntu ones, and changes that would only affect macOS (mostly just script changes I think) are relatively rare and can be caught quickly by those of us who run Macs.

[benalleng]

Following through that we don't really need these, after testing in #888 and with redis and docker removed the differences are just really minor and not worth the upkeep of these runners