Tests: Add unit test coverage for remove_menu_page() and remove_submenu_page()

[huzaifaalmesbah]

Description

Adds tests for remove_menu_page() and remove_submenu_page(), which had no coverage. The tests cover the returned menu item, removing only the first matching item while keeping the positions of the others, leaving the submenu (or parent menu) untouched, the empty menu, and the cases that must return false without changing anything: unknown, empty, partial and differently cased slugs, a submenu slug passed as a menu slug, a missing parent, and loosely equal values such as the integer 123 for the slug '123'.

The $menu and $submenu globals are saved in set_up() and restored in tear_down().

This is a test-only change; no production code is modified.

Testing Instructions

1. Run the tests on single site:
   npm run test:php -- --filter 'Tests_Admin_Includes_Plugin_RemoveMenuPage'
2. Run them on multisite:
   npm run test:php -- --filter 'Tests_Admin_Includes_Plugin_RemoveMenuPage' -c tests/phpunit/multisite.xml
3. Run them in random order:
   npm run test:php -- --filter 'Tests_Admin_Includes_Plugin_RemoveMenuPage' --order-by=random
4. Run the whole group to confirm nothing else is affected:
   npm run test:php -- --group plugins
5. Run PHPCS on the test file:
   node ./tools/local-env/scripts/docker.js run --rm php ./vendor/bin/phpcs tests/phpunit/tests/admin/includes/plugin/removeMenuPage.php

Trac ticket: https://core.trac.wordpress.org/ticket/65819

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Brainstorming edge-case scenarios and drafting test boilerplate; test architecture, assertions, and local PHPUnit verification were written and validated by me.

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