More focused test for post_type in tag cloud links.

We should not be testing the entire tag cloud markup, as it leads to collateral
damage wheneven `wp_generate_tag_cloud()` is touched. See [35984].

See #25566, #35138.
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git-svn-id: https://core.svn.wordpress.org/trunk@35952 1a063a9b-81f0-0310-95a4-ce76da25c4cd