[meta] Fix strict type errors in functional tests

Problem/Motivation

This is a child issue of #3376057: [META] Add declare(strict_types = 1) to all tests. After adding enabling strict types to all tests there were around 3000 errors. Fixing them all in one issue will lead to an enormous merge request that's difficult to review, as per the issue scope guidelines.

Steps to reproduce

See #3399746: Add declare(strict_types=1) to all Functional tests

Run the test suite:

./vendor/bin/phpunit -c core/phpunit.xml.dist --bootstrap=core/tests/bootstrap.php --testsuite=functional

Proposed resolution

Remaining tasks

• #3404273: Fix strict type errors: Convert FormattableMarkup to strings (simple replacement) in core Functional tests
• Fix strict type errors: Convert FormattableMarkup to strings (complex replacement) in core Functional tests
• Add casts and fix types where needed
• #3399746: Add declare(strict_types=1) to all Functional tests

User interface changes

API changes

Data model changes

Release notes snippet