bread-utils: fix flaky test isolation around process-global env vars

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).

bread-utils/src/hypr.rs

@@ -217,6 +217,7 @@ mod tests {
     // vars would be flaky.
     #[test]
     fn socket_path_env_var_behavior() {
+        let _lock = crate::env_test_lock().lock().unwrap_or_else(|e| e.into_inner());
         unsafe { env::remove_var("HYPRLAND_INSTANCE_SIGNATURE") };
         assert!(socket_path(Socket::Request).is_none());