Privacy: Add unit tests for wp_schedule_delete_old_privacy_export_files()

[roshniahuja]

Trac ticket: https://core.trac.wordpress.org/ticket/59707

Adds unit-test coverage for wp_schedule_delete_old_privacy_export_files(), exercising all three branches of the function:

• Schedules the wp_privacy_delete_old_export_files event when it is not already scheduled.
• Skips scheduling while WordPress is installing (wp_installing()).
• Does not create a duplicate event when one is already scheduled.

This builds on the work in #5550 and incorporates the review feedback from that PR:

• Calls parent::set_up() / parent::tear_down().
• Moves the wp_installing( false ) reset into tear_down() so it always runs.
• Adds the "already scheduled" branch test.
• Adds a standard file docblock, @group privacy, and assertion failure messages.

Verified locally against trunk: OK (3 tests, 5 assertions); PHPCS clean.

Note: this is an alternative to #5550 for the same ticket — happy to fold it back into that PR instead if maintainers prefer.

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