GH Actions: ensure zend assertions is enabled

Similar to the change made in commit antecedent@2657911 , `zend.assertions` is set to `-1` by default, so needs to be turned on by setting it to `1` for the tests to work.

Ref: shivammathur/setup-php#450

.github/workflows/tests.yml

@@ -28,7 +28,7 @@ jobs:
       with:
         php-version: ${{ matrix.php-versions }}
         extensions: mbstring
-        ini-values: error_reporting=-1, display_errors=On, log_errors_max_len=0
+        ini-values: zend.assertions=1, error_reporting=-1, display_errors=On, log_errors_max_len=0
         coverage: none
         tools: none