GH Actions: set error reporting to E_ALL

Turns out 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 `E_ALL` and `display_errors=On` to ensure **all** PHP notices are shown.

.github/workflows/test.yml

@@ -57,6 +57,7 @@ jobs:
         uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php }}
+          ini-values: error_reporting=E_ALL, display_errors=On
           coverage: ${{ steps.set_cov.outputs.COV }}
           tools: cs2pr