GH Actions: set error reporting to -1

The default setting for `error_reporting` used by the SetupPHP action is `error_reporting=E_ALL & ~E_DEPRECATED & ~E_STRICT` and `display_errors` is set to `Off`.

For the purposes of CI, I'd recommend running with `-1` and `display_errors=On` to ensure **all** PHP notices are shown.

Without this setting PHP 8.1 deprecation notices are not shown.

.github/workflows/tests.yml

@@ -26,6 +26,7 @@ jobs:
       with:
         php-version: ${{ matrix.php-versions }}
         extensions: mbstring
+        ini-values: error_reporting=-1, display_errors=On, log_errors_max_len=0
 
     # Install dependencies and handle caching in one go.
     # @link https://github.com/marketplace/actions/install-composer-dependencies