Add an experimental CI job for PHP 8.4

Author: stof

.github/workflows/unit-tests.yml

@@ -33,7 +33,8 @@ jobs:
           - php: '8.2'
             mode: low-deps
           - php: '8.3'
-            #mode: experimental
+          - php: '8.4'
+            mode: experimental
       fail-fast: false
 
     runs-on: ubuntu-20.04