Ensure test file is cleaned up in cwd_is_writable test case

tests/integration.rs

@@ -9,8 +9,18 @@ fn sandbox(extra_args: &[&str]) -> Command {
     cmd
 }
 
+struct CleanupFile(&'static str);
+
+impl Drop for CleanupFile {
+    fn drop(&mut self) {
+        let _ = fs::remove_file(self.0);
+    }
+}
+
 #[test]
 fn cwd_is_writable() {
+    let _cleanup = CleanupFile("./sandbox_canary");
+
     let output = sandbox(&[])
         .args(["--", "bash", "-c", "touch ./sandbox_canary && echo ok"])
         .output()