I would just like to start by saying I am not asking what causes a zombie to turn a villager or anything like that.
I was wondering, what are the chances of a Zombie Testificate to spawn naturally in Minecraft, and when do they spawn? I have most often found they tend to spawn near dawn (which is frustrating attempting to kite them around if I want to try and capture them).
So to summarize:
According to the Minecraft Wiki page on zombies:
In the Overworld, zombies spawn in groups of 4 at a light level of 7 or less. Zombies have a 5% chance to spawn as a zombie villager and a 5% chance to spawn as a baby zombie. Baby zombies have an additional 5% chance of spawning as a chicken jockey.
This means that they have a 5% chance to spawn at any point in the night.
If an Zombie spawns at any point naturally (or from an Zombie Spawner), they have an 5% chance to spawn as an Zombie Testificate.
Zombie villagers have a 5% chance of naturally spawning. (Also, baby zombie villagers will be spawned as 5% of their adult counterparts, resulting in a spawn chance of 0.25% natural spawn chance.) Also, remember the under 8 light level spawning condition. (Also seen in another answer). As for the thing that they burn in the sunlight, you can throw a splash potion of fire resistance with long effect (as a last resort). For more about zombies, see the wiki.
