[TESTING ONLY; DO NOT COMMIT] Update to PHPUnit 9

Created on 4 June 2021, almost 4 years ago

Updated 4 March 2024, about 1 year ago

Problem/Motivation

Our composer.lock currently locks to PHPUnit 8, and Drupal\Core\Composer\Composer::upgradePHPUnit() only upgrades to PHPUnit 9 if we're on PHP 7.4.
However, according to https://phpunit.de/supported-versions.html, PHPUnit 9 is supported on PHP 7.3, so I don't know if/why we need to lock to PHPUnit 8 on PHP 7.3.
Meanwhile, per the above link, PHPUnit 8 is EOL ("Support" ended in Feb 2021 and "Life support" does not cover bug fixes for PHPUnit on PHP 7.3).
We're still running old versions of PHPUnit for Drupal 8 until Nov 2021, but Drupal 9.2 will be supported until June 2022 and Drupal 9.3 will be supported until Dec. 2022, so we'd get some benefit from dropping PHPUnit 8 for either or both of those version.
It might be too late for Drupal 9.2, so setting the version of this issue to 9.3 for now.

Steps to reproduce

Proposed resolution

This patch is created by running:

composer update phpunit/phpunit --with-dependencies
composer require --dev phpspec/prophecy-phpunit:^2

, which are essentially the same commands currently called in the drupal-phpunit-upgrade composer script of HEAD's root composer.json file, and also adding "nikic" (a dependency of PHPUnit 9) to core/misc/cspell/dictionary.txt per the interdiff in #3.