Added test for new isValidExpression() static function.
Added test for new isValidExpression() static function.

williamorazi/cron-expression,Marketing1by1/cron-expression,OFFLINE-GmbH/cron-expression,mtdowling/cron-expression,kbedn/cron-expression,juliangut/cron-expression,Moayadkh/cron-expression,OVYA/cron-expression