`helix_tui::widgets::Cell` contains styled `Text`. In the case of a
styled `Cell` with styled `Text`, the styled `Text` would override the
style of the `Cell`. This led to errors where it seemed
`ui.text.directory` had more priority than `ui.menu.selected` and
`ui.text.focus`. This change also adds some tests from doc comments of
`Text` and makes them actually tested.
Closes: #13768
Caleb Larsen