Fix test suite isolation by cleaning up registered theme directories and cache in teardown #895

Running the full PHPUnit test suite (npm run test:unit:php) produces 1 error and 5 failures across Test_Create_Block_Theme_Api and Test_Create_Block_Theme_Fonts, even though both test classes pass completely when run in isolation.

This happens because CBT_Theme_Locale_UnitTestCase (tests/CbtThemeLocale/base.php) and CBT_Theme_Readme_UnitTestCase (tests/CbtThemeReadme/base.php) register a custom test theme directory via register_theme_directory() in set_up(), but do not restore the global $wp_theme_directories array or call wp_clean_themes_cache() in tear_down().

Steps to Reproduce

1. Run CBT_Theme_Locale_EscapeAttribute immediately followed by Test_Create_Block_Theme_Api:

   npm run test:unit:php:base -- --filter "CBT_Theme_Locale_EscapeAttribute|Test_Create_Block_Theme_Api"

2. Observe 2 failures in Test_Create_Block_Theme_Api.

3. Run Test_Create_Block_Theme_Api alone:

   npm run test:unit:php:base -- --filter Test_Create_Block_Theme_Api

4. Observe that all tests pass.

Expected Behaviour

Tests should run in clean isolation. Base test cases that register custom theme directories should restore $wp_theme_directories and call wp_clean_themes_cache() in tear_down() so subsequent test suites are not polluted.

Actual Behaviour

Custom theme directories remain in the global $wp_theme_directories array and theme caches remain stale. Subsequent tests that dynamically create themes on disk fail to resolve them, leading to 1 error and 5 failures during full suite runs.