Posts, Post Types: Simplify `test_setup_postdata_loop()`.

The important part here is calling `the_content()` after setting up post data for another post without updating global `$post`.

The `foreach()` loop is not necessary.

Follow-up to [UT1289].

See #47824, #24330.
Built from https://develop.svn.wordpress.org/trunk@48113


git-svn-id: https://core.svn.wordpress.org/trunk@47882 1a063a9b-81f0-0310-95a4-ce76da25c4cd