Tests: Refactor test_wp_cache_get_multiple() and test_wp_cache_delete_multiple()

[jorgelr771]

Trac ticket:
https://core.trac.wordpress.org/ticket/65693

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[Copilot]

Pull request overview

This PR refactors and strengthens the unit tests covering wp_cache_get_multiple() and wp_cache_delete_multiple() to better validate group isolation and expected behavior for missing keys, aligning with the cache API’s documented return values.

Changes:

• Expand test_wp_cache_get_multiple() to cover: keys missing in the requested group, keys present only in another group, and same key stored in two groups with different values.
• Expand test_wp_cache_delete_multiple() to include a missing key, and add post-conditions verifying deletions actually occurred only within the targeted group.

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