Skip Linux-only supervisor tests on non-Linux / missing deps
The systemd and runit installer tests exist to exercise real system
integration, but the two tests fail hard on macOS and on Linux boxes
that dont have runit installed — not because the code is broken,
just because the required binaries arent there. Converted the
runtime-throw (runsvdir case) and the platform-mismatch (systemd
case) into it.skipIf gates so developers on Mac see "skipped"
instead of "runsvdir is not installed" red errors.
CI on Linux still runs both; its where the real validation lives.