Feature: Add Test for is_blog_installed

[Debarghya-Banerjee]

Trac Ticket: Core-54754

Summary

This PR adds a series of unit tests for the is_blog_installed() function in WordPress. The is_blog_installed() function checks if the WordPress installation is complete by verifying the existence of necessary options and tables in the database.

The tests ensure that the function behaves as expected under different conditions, including the presence or absence of the siteurl option, the state of the database tables, and certain WordPress constants like WP_REPAIRING.

Benefits

• Coverage for Default Behavior: The tests ensure that is_blog_installed() behaves correctly in various scenarios, including default behavior, when siteurl is set or empty.

• Edge Cases: The tests also account for edge cases such as the absence of siteurl, missing database tables, and when WordPress is in repair mode.

• Performance Assurance: The caching test ensures that once the installation status is determined, it is cached for faster subsequent checks, improving performance.

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