Tests: Add enable_real_time_collaboration to REST settings test

[pkevan]

Summary

• Adds enable_real_time_collaboration to the expected settings array in WP_Test_REST_Settings_Controller::test_get_items
• The new setting is registered with show_in_rest in Real-time collaboration: Add new REST endpoints, setting, and registered post meta #10894 but was missing from the test hardcoded expected list, causing test failures across all PHP/DB matrix combinations

Test plan

• CI passes for WP_Test_REST_Settings_Controller::test_get_items

Fixes test failures in #10894

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Props czarate, paulkevan.

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