In terms of plone.app.testing fixtures, "acceptance tests" means tests that can commit transactions but still get rolled back to their starting point after each test -- this can be used for Robot Framework tests but also other tests.
I haven't written Robot Framework tests in years (with Volto projects we are usually using Cypress) so I don't have advice to offer from experience on how to use Robot Framework with pytest. I expect you would need a pytest plugin to discover Robot Framework tests.