clickButton should match (or prefer) full-text matches over substring matches

```html

A
Analyze

```

```elm
ProgramTest.clickButton "A"
```

This should be able to pass, clicking the first button.

This is a shortcoming of https://package.elm-lang.org/packages/elm-explorations/test/latest/Test-Html-Selector#text afaik, `Test.Html.Selector` provides no way to distinguish full-text matches from substring matches, so I'm not sure there's a way to fix this without additions to `elm-explorations/test`.

Start by reproducing the HTML example with ProgramTest.clickButton "A" and inspect how it uses Test.Html.Selector.text. Check whether the selector dependency can distinguish full-text from substring matches; done means the first button is selected without breaking existing clickButton behavior, or the dependency limitation is documented.

Written by the indexing model from the issue text.