Add some basic build instructions to run unit tests

[sgryphon]

These build instructions were reverse engineered out of the github actions (diving into arduino/cpp-test-action).

I think the dependencies are correct, but my machine already had a bunch of software installed, so I'm not sure (but it is a start).

This is useful for someone who wants to contribute to the repo, to run the host-based tests locally (before submitting).

(Note: I also had to make a minor change in catch.hpp, to set sigStackSize = 32768, as MINSIGSTKSZ was no longer defined (some kind of version incompatibility). I have not checked that change in.)

[aentinger]

Hi @sgryphon ☕ 👋 Thank you very much for your contribution! I added a couple of commits containing minor improvements, but those are largely simplifications.