test: add cd is builtin test

src/builtins/mod.rs

@@ -14,3 +14,13 @@ pub fn execute_builtin(keyword: &str, args: &[&str]) -> Result<(), ShellError> {
     }
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_is_builtin_cd() {
+        assert_eq!(is_builtin("cd"), true);
+    }
+}