`css.properties.text-box-edge.cap/ex` tests check invalid value

[caugner]

The tests for css.properties.text-box-edge.cap/ex check CSS.supports('text-box-edge', 'cap/ex'), but cap and ex alone are not valid values for <text-edge>:

<text-edge> = [ text | ideographic | ideographic-ink ]
              | [ text | ideographic | ideographic-ink | cap | ex ]
                [ text | ideographic | ideographic-ink | alphabetic ]

Instead, tests would need to check CSS.supports('text-box-edge', 'cap/ex text/ideographic/ideographic-ink/alphabetic').

Ideally this could be derived automatically from the definition, or else we need custom tests.

Noticed via mdn/browser-compat-data#29325.