New default themes require workarounds being added to several unit tests, as they often alter default WordPress behaviour. To avoid ongoing maintenance issues, this change switches to a minimal theme when running tests.
This change also removes the old workarounds for default themes.
Fixes#31550.
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git-svn-id: http://core.svn.wordpress.org/trunk@38801 1a063a9b-81f0-0310-95a4-ce76da25c4cd