faketime issues on MacOS

[jku]

faketime apparently does not work on MacOS (the OS prevents library loading tricks that faketime needs): this is not a massive issue (see below for mitigations) but it is annoying for the developers who have decided to use a Mac -- running tests locally gets tricky.

Options to solve this:

1. document and skip tests locally

   • document that the complete test suite only runs on linux (the action already requires linux in practice)
   • skip any faketime tests on non-linux platforms (not ideal but at least developers could run other tests locally)

2. Stop using faketime

   • Add a --time <unix-timestamp> option to the client-under-test CLI: This requires changes in every client so is a little unappealing
   • this way faketime would not be required by test suite: instead the client could choose to use faketime or another method of changing "system time"

[jku]

CC @aaronlew02, @loosebazooka -- I'm not planning on working on this now but if you have ideas leave them here