Update workflow: Make phpunit experimental for PHP8.4

from miraheze/.github (miraheze/.github@a48c363)

Author: Universal-Omega

.github/workflows/mediawiki-tests.yml

@@ -28,7 +28,7 @@ jobs:
             composer-test: true
             coverage: false
             experimental-phan: false
-            experimental-phpunit: false
+            experimental-phpunit: true
 
           # Latest MediaWiki stable - PHP 8.2
           - mw: 'REL1_44'
@@ -46,7 +46,7 @@ jobs:
             composer-test: false
             coverage: false
             experimental-phan: true
-            experimental-phpunit: false
+            experimental-phpunit: true
 
           # Latest MediaWiki master - PHP 8.2
           - mw: 'master'