Build/Test Tools: When testing the output of `wp_list_pages()`, use a known...

Build/Test Tools: When testing the output of `wp_list_pages()`, use a known and fixed date for each post so the tests don't fail when the date changes between the beginning and end of a test.

Props xrmx
Fixes #38688

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git-svn-id: https://core.svn.wordpress.org/trunk@39303 1a063a9b-81f0-0310-95a4-ce76da25c4cd