After [28883], ensure that priorities have callbacks before returning `true` in `has_filter()`.

Adds unit tests.

Props boonebgorges.
Fixes #29070.

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`WP_List_Table::items` should be `public`, not `protected`. It is accessed in a subclass directly via `wp_ajax_get_comments()`.

See [28493].
Fixes #29072.

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git-svn-id: http://core.svn.wordpress.org/trunk@29201 1a063a9b-81f0-0310-95a4-ce76da25c4cd
After [29201], use `aria-label` instead of `aria-labelledby` in `WP_Plugin_Install_List_Table::display_rows()`.

Props nvwd, DrewAPicture.
Fixes #28853.

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git-svn-id: http://core.svn.wordpress.org/trunk@29202 1a063a9b-81f0-0310-95a4-ce76da25c4cd