consider relocating tests for `tune_args()` and `tunable()`

[simonpcouch]

As an additional comment... there are tests for tune_args() and tunable() methods in extratests:

• https://github.com/tidymodels/extratests/blob/main/tests/testthat/test-tune_args.R
• https://github.com/tidymodels/extratests/blob/main/tests/testthat/test-tunable.R

do you think it's worth consolidating them with the tests here?

Originally posted by @hfrick in #193 (review)

[DavisVaughan]

I think they are fine here because you only need workflows Imports/Suggests to test them and they are specific to workflows itself

[hfrick]

I think we needed the tests to be in extratests during the transition period of the various methods (or at least it was easier). I meant more "moving tests into workflows" rather than the other direction - if there's anything in those tests in extratests that isn't covered here already.