Support for elm-ui [tracking issue]

Ideally elm-program-test's [functions for simulating user input](https://package.elm-lang.org/packages/avh4/elm-program-test/latest/ProgramTest#simulating-user-input) should work with HTML produced by [mdgriffith/elm-ui](https://package.elm-lang.org/packages/mdgriffith/elm-ui/latest/). So long as that HTML follows accessibility best practices, we should add support for it.

**If you are using elm-ui, and** a user input functions in **elm-program-test doesn't work** in some particular case, **please make a new issue** with the example rendered HTML and which function you're expecting to work ([example issue](https://github.com/avh4/elm-program-test/issues/27)).

[Current issues](https://github.com/avh4/elm-program-test/labels/elm-ui)

Start with the elm-program-test documentation for functions that simulate user input and review the linked current issues labeled elm-ui. Look for concrete child issues containing rendered HTML and an expected function behavior. Done means the relevant user-input functions work with accessible HTML produced by mdgriffith/elm-ui.

Written by the indexing model from the issue text.