test: add exit test

src/builtins/exit.rs

@@ -8,3 +8,13 @@ impl Builtin for Exit {
         Err(ShellError::Exit)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_exit() {
+        assert_eq!(Exit.execute(vec!["arg".to_string()]), Err(ShellError::Exit))
+    }
+}