Test: fix, hopefully, false PSYNC failure like in issue #2715.
Test: fix, hopefully, false PSYNC failure like in issue #2715.

And many other related Github issues... all reporting the same problem.
There was probably just not enough backlog in certain unlucky runs.
I'll ask people that can reporduce if they see now this as fixed as
well.

pedigree/redis,pedigree/redis,pedigree/redis,pedigree/redis