Canonical: Move trailing slash handling for `robots.txt` and sitemaps to a single condition.

Give the unit test a more descriptive name.

Follow-up to [48153], [48155].

See #48025.
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git-svn-id: https://core.svn.wordpress.org/trunk@47935 1a063a9b-81f0-0310-95a4-ce76da25c4cd