Add support for interactive with/asserting on table cells

See https://webaim.org/techniques/tables/data

Read the issue title and the linked WebAIM table guidance first to clarify the expected interactive behavior for table cells. Then locate the existing Elm program-test APIs for interaction and assertions; done means table-cell interactions and assertions are supported with tests demonstrating the intended behavior, but the issue does not name specific files or tests.

Written by the indexing model from the issue text.