WordPress+ActivityPub outgoing message sending is unreliable.

Consider this test:

1. Mastodon account follows WordPress account
2. WordPress account posts something
3. test that Mastodon account receives it

It appears that on the WordPress side, the ActivityPub HTTP POST of the Create/Note activity triggered by step 2 runs with `wp-cron` (which I don’t really understand, except that I think it has some kind of queue that is worked down as a side effect of incoming HTTP requests).

If I suspend the automated test run after step 2, and use a browser to access the WordPress instance, it appears the HTTP POST to the Mastodon inbox is performed. If it runs in unattended mode, the POST is not performed.

I tried to `curl https://wordpress/wp-cron.php` and `curl https://wordpress/wp-cron.php?doing_wp_cron` but that doesn’t seem to trigger it. If I run it once every second for 15 seconds, it triggers it most of the time, but not always.

Find a way to get WordPress to reliably deliver that POST.

See also:
* https://wordpress.org/support/topic/activitypub-message-sending-time-constants/
* https://wordpress.org/support/topic/reliable-create-note-delivery-when-scripting/