Add automated test for "Clear All"

The manual test is here: Clear All

Existing automated coverage

• NotificationsIT#simpleNotifications: tray.clearAllNotifications() opens the bell, clicks "Clear All" then Yes and waits for the "Notifications have been cleared" message. waitOnNotificationCount(0) then reloads the page, and the test asserts the .noitems ("No notifications available.") element and a count of 0, so the cleared state after a refresh is covered.
• NotificationsIT#compositeNotifications: Also uses Clear All before and after its scenario.

Missing automated coverage

• Before Clear All, assert that the unread badge (span.notifications-count) is displayed (needs a page-object method such as NotificationsTrayPage#isCountBadgeDisplayed).
• After Clear All, assert that the badge is no longer displayed.
• After reloading the page, assert that the badge is still absent.

Suggested location: xwiki-platform-core/xwiki-platform-notifications/xwiki-platform-notifications-test/xwiki-platform-notifications-test-docker/src/test/it/org/xwiki/platform/notifications/test/ui/NotificationsIT.java

Found by comparing the XWiki 18.4.4 manual test runs against the automated tests on master (a0055f10a1a).