new_audit: td-has-header

[jazyan]

See http://go/prcpg for the current list of audits to be enabled.

Surprisingly, a11y_tester.html already has a specific test case for this.

[connorjclark]

This audit has some funny history.

#1768 added the test case.

#6169 is when Paul noticed we never even had an audit for it.

[connorjclark]

Can you fill out the expected audit result for td-has-header in cli/test/smokehouse/test-definitions/a11y.js? Ran via yarn smoke a11y

[paulirish]

fwiw This is the test failure I'm seeing in the CI logs:

[jazyan]

Added td-has-header case to a11y smoke tests, and updated the category renderer unit test.

Looking into the DevTools e2e failing assertions 👀

(and whoops, accidentally re-added commits after fetch from main.)

[brendankenny]

just some potential UIStrings suggestions but otherwise LGTM!