Your server might be blocking requests that look suspicious.
Check your server logs and see if POST from Stripe is not getting blocked by the server config. Here’s how the log entry looks in my Nginx server access log:
POST /?asp_hook=test HTTP/1.1″ 200 0 “-” “Stripe/1.0 (+https://stripe.com/docs/webhooks)
Also, do you have any WP security or firewall type plugins installed on your site? If yes, can you disable those and try to test the webhook again?