Same as 5e0b1aeb, for the companion test file.

This one seems lower probability (only two failures in a month of runs);
I was hardly able to reproduce a failure without a patch, so the fact
that I was also unable to reproduce one with it doesn't say anything.
We'll have to wait for further buildfarm results to see if we need any
further adjustments.

Discussion: https://postgr.es/m/20210524090712.GA3771394@rfd.leadboat.com