Fix deprecated env::set_var/remove_var in tests

Wrap all env::set_var and env::remove_var calls in unsafe blocks
(deprecated as unsafe since Rust 1.81, will error on edition 2024).

Add serial_test crate and #[serial] attribute to all tests that
mutate environment variables to prevent parallel execution races.

Fix XDG_CONFIG_HOME leak between tests by unsetting it in
set_config_dir() helper.

Tests now pass reliably with multi-threaded execution.

Author: simeg

Cargo.lock

@@ -31,4 +31,5 @@ serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 
 [dev-dependencies]
+serial_test = "3"
 tempfile = "3"