[TASK] Update PHPunit to v9

PHPunit v9 allows to use further improvements to our testing
API, and also offers extended support until at least Feb 2022.

PHPunit v9 requires PHP 7.3+ which isn't a problem
for us per-se.

In addition, composer does not complain about
an abandoned package anymore if we upgrade to PHPunit 9.

In order to achieve this, we need to explicitly
require "codeception/phpunit-wrapper:^9.0" which we
can then remove later-on again.

Used composer commands:
> composer req --dev "phpunit/phpunit:^9.5" "phpspec/prophecy-phpunit" "codeception/phpunit-wrapper:^9.0" -W
> composer req --dev "phpunit/phpunit:^9.5" "phpspec/prophecy-phpunit" -d typo3/sysext/core --no-update
> composer remove --dev codeception/phpunit-wrapper

Used rector to migrate all Unit tests to prevent warnings..

Resolves: #94706
Releases: master
Change-Id: I68d9df07421149f4ad40964afc512f0f995b295b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69297


Tested-by: core-ci <typo3@b13.com>
Tested-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>