Test: Don't skip the tests intended for single site when running on Multisite,...

Test: Don't skip the tests intended for single site when running on Multisite, add them to the `ms-excluded` group instead.

See #39776, #45747.
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git-svn-id: https://core.svn.wordpress.org/trunk@46483 1a063a9b-81f0-0310-95a4-ce76da25c4cd