Bug 1872954 - [Protections Panel] Ensure blocker tracker milestones subviewbutton is labeled. r=anti-tracking-reviewers,pbz
The Protections doorhanger UI includes [a subview button with id `protections-popup-milestones-content`](https://searchfox.org/mozilla-central/rev/762f24e00a9548d80ebba1b985c871ba6d9b829d/browser/components/controlcenter/content/protectionsPanel.inc.xhtml#160-167) that does not provide an accessible name,
It appears to happen because this `<toolbarbutton>` XUL control is marked up with [`<description>` as its child containing the on-screen text](https://searchfox.org/mozilla-central/rev/762f24e00a9548d80ebba1b985c871ba6d9b829d/browser/components/controlcenter/content/protectionsPanel.inc.xhtml#164-165), while `<label>` does provide an expected output for the Accessibility API preserving its visual appearance at the same time.
Since this control would become labeled and would start passing a11y_checks, we are also removing [the `fail-if` notation](https://searchfox.org/mozilla-central/rev/762f24e00a9548d80ebba1b985c871ba6d9b829d/browser/base/content/test/protectionsUI/browser.toml#57) from the `browser/base/content/test/protectionsUI/browser_protectionsUI_milestones.js` test manifest.
Note: the following prefs are needed:
```
// Hide protections cards so as not to trigger more async messaging
// when landing on the page.
["browser.contentblocking.report.monitor.enabled", false],
["browser.contentblocking.report.lockwise.enabled", false],
["browser.contentblocking.report.proxy.enabled", false],
["browser.contentblocking.cfr-milestone.update-interval", 0],
```
Differential Revision: https://phabricator.services.mozilla.com/D197634
