Tests: Copy themes from `tests/phpunit/data` to `wp-content/themes`, instead of creating a symlink.

This allows the theme file tests in `phpunit/tests/link/themeFile.php` to run on Windows without requiring administrative privileges.

Follow-up to [42812], [42819].

Props danielhuesken, christophherr, davidbaumwald, SergeyBiryukov.
See #40856, #39975.
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