test: add test is builtin for blahaj command

src/builtins/mod.rs

@@ -145,4 +145,9 @@ mod tests {
     fn test_is_builtin_notabuiltin() {
         assert!(!is_builtin("notabuiltin"))
     }
+
+    #[test]
+    fn test_is_builtin_blahaj(){
+        assert!(is_builtin("blahaj"));
+    }
 }