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/lib.rs

@@ -25,6 +25,18 @@ pub mod proc;
 pub mod singleton;
 pub mod xdg;
 
+/// Serializes tests that read or mutate process-global env vars
+/// (`XDG_RUNTIME_DIR`, `HYPRLAND_INSTANCE_SIGNATURE`) — `cargo test` runs
+/// tests in parallel threads within one process by default, and
+/// `std::env::set_var` is process-wide, so a `hypr` test temporarily
+/// pointing `XDG_RUNTIME_DIR` at a nonexistent path can otherwise race a
+/// concurrently-running `singleton` or `xdg` test that expects the real one.
+#[cfg(test)]
+pub(crate) fn env_test_lock() -> &'static std::sync::Mutex<()> {
+    static LOCK: std::sync::OnceLock<std::sync::Mutex<()>> = std::sync::OnceLock::new();
+    LOCK.get_or_init(|| std::sync::Mutex::new(()))
+}
+
 #[cfg(feature = "toml")]
 pub mod tomlcfg;