Update "php-ext-install" test to make sure the extension is loaded properly too, not just installed

test/tests/php-ext-install/container.sh

@@ -3,3 +3,4 @@
 set -ueo pipefail
 
 docker-php-ext-install pdo_mysql 2>&1
+php -r 'exit(extension_loaded("pdo_mysql") ? 0 : 1);'