singleton.rs's tests used to call the real kill() on the current test
process itself (the recorded "other instance" PID was our own, since
tests run single-process) — split try_acquire (side-effect-free) out from
toggle_or_kill (sends the signal) so the concurrency assertions no longer
risk SIGTERM-ing the test binary.
Separately, hypr.rs's env-var test mutates HYPRLAND_INSTANCE_SIGNATURE/
XDG_RUNTIME_DIR process-globally; cargo runs tests in parallel threads by
default, so it could race a concurrently-running singleton or xdg test
expecting the real XDG_RUNTIME_DIR, intermittently failing them with
ENOENT. Added a shared env_test_lock() all env-var-touching tests now
acquire for their duration.
Verified via 5 repeated full test runs with zero flakes (28/28 passing
each time).
49c63c8ccf