Tests: Improve the On This Day dashboard widget tests

[mukeshpanchal27]

Refactor tests to use static user IDs for consistency and clarity.

Trac ticket: https://core.trac.wordpress.org/ticket/65116

Follow-up to https://core.trac.wordpress.org/changeset/62681, which added the On This Day dashboard widget along with its test coverage. This PR is a test-only maintenance pass on Tests_Admin_wpOnThisDay:

• Adds the missing @ticket 65116 annotation to each test method, so the tests are discoverable via the ticket group as required by the core test conventions.
• Moves the repeated author user creation into shared wpSetUpBeforeClass() fixtures (self::$user_id and self::$other_user_id), replacing the per-test self::factory()->user->create() calls. The two users now carry the Current Writer / Guest Writer display names that test_widget_labels_posts_from_other_authors() previously created inline, so every test shares one consistent set of authors.

No production code is touched and no test assertions change; this only reduces duplicated setup and the number of users created per test run.

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Pull request overview

This PR is a test-only maintenance refactor for the On This Day dashboard widget PHPUnit tests, aiming to reduce duplicated setup and make author fixtures consistent across the test class.

Changes:

• Added shared author fixtures (self::$user_id, self::$other_user_id) in wpSetUpBeforeClass() and updated tests to reuse them.
• Added @ticket 65116 annotations to the relevant test methods for discoverability/grouping by ticket.

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Props mukesh27, westonruter.

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