Bundled Themes: Fix fragile `update-theme` phpunit test which broke after...

Bundled Themes: Fix fragile `update-theme` phpunit test which broke after version updates in [42105].

See #42090.

Built from https://develop.svn.wordpress.org/trunk@42106


git-svn-id: https://core.svn.wordpress.org/trunk@41935 1a063a9b-81f0-0310-95a4-ce76da25c4cd