test: add sus is builtin command test

src/builtins/mod.rs

@@ -95,6 +95,11 @@ mod tests {
         assert_eq!(is_builtin("change-prompt"), true);
     }
 
+    #[test]
+    fn test_is_builtin_sus() {
+        assert_eq!(is_builtin("sus"), true);
+    }
+
     #[test]
     fn test_is_builtin_notabuiltin() {
         assert_eq!(is_builtin("notabuiltin"), false)