Upkeep for `test-recipes-varying.R`

`varying()` has been replaced by `tune()` and `varying_args()` by `tune_args()` (which has tests in `test-tune_args.R`).

It'd be good to check if any of the tests here should be updated (and possibly migrated to live either in `test-tune_args.R` or in recipes) or if they could be deleted.