Trial spawners only check if parent slimes/magma cubes are killed

When fighting a slime or magma cube trial spawner, the spawner checks only if the originally spawned slimes are killed when advancing, and not the smaller slimes spawned when the bigger trial slimes are killed. This can lead to the spawner prematurely spawning new slimes or giving a reward and usually causes slime trials to end with many tiny slimes, and even some medium slimes, still alive when the trial is complete.

Steps to reproduce:

1. Find a slime trial spawner or place one in creative

2. Set gamemode to Survival

3. Start killing the slimes as they spawn

4. When a slime is killed, completely ignore all smaller slimes summoned by the dead slime 

Expected result: The trial spawner waits until all slimes are killed before dropping a reward and going on cooldown.

Actual Result: The trial spawner ignores the smaller slimes and produces a reward anyway once the last spawner-created slime is killed, despite the fact that slimes originating from the spawner are still alive.