asynctest: hold stop() and start() of a listen event to a pause (true-async/php-async#305)

[EdmondDantes]

Five cmocka cases for the listen event's pause, which true-async/php-async#305 implements:

• a stopped listener accepts none of three connections until start(), and delivers them on the ticks after it, none from inside start();
• a held connection survives a second pause;
• a callback that pauses during delivery stops it;
• dispose while paused, and dispose between start() and the delivery tick, release every handle.

All five fail against php-async main (e580c15) and pass against its listen-pause branch, 3 of 3 runs. Built with --enable-asynctest; no CI job runs this SAPI.