Tests: Add unit tests for wp_using_themes()

[shail-mehta]

What

Trac ticket: #65819

Adds unit tests for wp_using_themes() in tests/phpunit/tests/load/wpUsingThemes.php.

Covers WP_USE_THEMES behavior and the wp_using_themes filter, including override when the constant is true.

Test-only change.

Why

This helper controls whether themes should load for the current request and has had no dedicated PHPUnit coverage.

How

• Cover undefined / true / false WP_USE_THEMES and filter overrides

Testing instructions

npm run test:php -- --filter Tests_Load_WpUsingThemes

Use of AI Tools

• Cursor

[Copilot]

🟢 Approval recommended

The tests cover the stated behavior with appropriate constant isolation, and no blocking issues were identified.

No open issues 🎉

What changed in this PR

Adds dedicated PHPUnit coverage for wp_using_themes(), with no production code changes.

Changes:

• Tests undefined, true, and false WP_USE_THEMES states.
• Tests filter overrides, including when the constant is true.
• Isolates tests that define the constant in separate processes.

File	Description
tests/​phpunit/​tests/​load/​wpUsingThemes.php	Adds five tests for constant behavior and filter overrides.

_{Review effort: Balanced}

[github-actions]

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