Tests: Add unit tests for wp_admin_canonical_url()

[pbearne]

This ticket adds comprehensive unit tests for the wp_admin_canonical_url() function in wp-admin/includes/misc.php. The tests will cover the default URL generation, handling of query parameters, and the wp_admin_canonical_url filter.

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Description:
This PR adds unit tests for the wp_admin_canonical_url() function in wp-admin/includes/misc.php. These tests ensure that the function correctly identifies and removes single-use query parameters (such as settings-updated, trashed, etc.) from the administration URL to generate a clean canonical link.

The tests cover:

• Standard canonical URL generation for common admin screens.
• Successful removal of one or more "removable" query arguments.
• Correct handling of HTTPS URLs.
• Verification that no output is generated if the list of removable arguments is empty (via filter).
• Verification of the wp_admin_canonical_url filter for custom canonical URLs.

Trac ticket: https://core.trac.wordpress.org/ticket/65192

AI Disclosure:

• AI assistance: Yes
• Tool(s): Junie (JetBrains)
• Model(s): gemini-3-flash-preview
• Used for: Code analysis, test implementation, and workflow management.

[desrosj]

Adding the tests focus, which is used to indicate a ticket is solely focused on adding tests.

[SergeyBiryukov]

In 62371:

Tests: Add unit tests for wp_admin_canonical_url().

Follow-up to [31736].

Props pbearne.
Fixes #65192.