test: add exit is builtin test

src/builtins/mod.rs

@@ -21,6 +21,11 @@ pub fn execute_builtin(keyword: &str, args: &[&str]) -> Result<(), ShellError> {
 mod tests {
     use super::*;
 
+    #[test]
+    fn test_is_builtin_exit() {
+        assert_eq!(is_builtin("exit"), true);
+    }
+
     #[test]
     fn test_is_builtin_cd() {
         assert_eq!(is_builtin("cd"), true);