test: add is not a builtin test

src/builtins/mod.rs

@@ -38,4 +38,9 @@ mod tests {
     fn test_is_builtin_cd() {
         assert_eq!(is_builtin("cd"), true);
     }
+
+    #[test]
+    fn test_is_builtin_notabuiltin() {
+        assert_eq!(is_builtin("notabuiltin"), false)
+    }
 }