minor: fix race condition in refresh test

Before 5ca92265ac89762de8585278ada0d3902c82468f our set consisted of two data nodes and an arbiter.

- Primary
- Secondary
- Arbiter

Now we have three data nodes so killing all secondaries causes the majority of the set to be down.

- Primary
- Secondary (X)
- Secondary (X)

As we know, this means the existing primary will step itself down once it notices the majority of the set is not up. It is just by coincidence that some of the refresh tests pass before that can occur. Sometimes that is not the case and we see an error during testing.

This pull request is meant to fix that problem.