Taxonomy: Make sure `wp_terms_checklist()` and...

Taxonomy: Make sure `wp_terms_checklist()` and `Walker_Category_Checklist::start_el()` properly handle an array of strings as `selected_cats` or `popular_cats` values.

Even with these values documented as an array of integers, they can technically also accept an array of strings, e.g. as form results.

Add a unit test.

Props brianhogg, TimothyBlynJacobs, SergeyBiryukov.
Merges [48880] to the 5.5 branch.
Fixes #51137.
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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48644 1a063a9b-81f0-0310-95a4-ce76da25c4cd