Symfony version(s) affected: 4.4-dev

Description
Our AppVeyor build is currently broken. Each test suite fails with the following error:

Composer detected issues in your platform:
Your Composer dependencies require the following PHP extensions to be installed: mbstring

The reason for that is a failed platform check enforced by Composer 2. That check for ext-mbstring is installed because PHPUnit explicitly asks for the extension:

https://github.com/sebastianbergmann/phpunit/blob/e936ac1a54a49d1636f05e6c8871f73a79b877f6/composer.json#L28

Of course, we could just enable the extension. But I believe, ext-mbstring among other extensions is missing here on purpose because we want to test Symfony with minimal extensions enabled.

How to reproduce
https://ci.appveyor.com/project/fabpot/symfony/builds/35947101

Possible Solution

• Enable ext-mbstring anyway.
• Find a way to get around the platform check (e.g. same hack as Disable platform checks #38707).