What I expected to happen was...:
They would sell mending eventually.

What actually happened was...:
I kept destroying and replacing the lectern, I used 1 1/4 iron axes while doing this. I got every other enchantment multible times. 

Edit: Actually, I kept continuing, and then I finally got it! I was just extremely unlucky for a long time...

Steps to Reproduce:
1. Cure 2 zombievillagers.
2. Breed them.
3. Wait for 1 of the offspring to grow up.
4. Make that offspring a liberarian.
5. Put the liberarian in a confined space with you and a lectern.
6. Check the villagers trades, destroy the lectern if the villager does not have the mending trade, put the lectern back and keep doing this for a long time.