Fix RCv3 response in test

This should be reviewed after #772, #773, #779.

TL;DR: the result of the RCv3 effect looks like: `[[{...}]]`. Outer list is added by `ParallelEffects`; inner list is actually part of the RCv3 response, although the list is always a singleton, since we only ever add/remove one node-LB connection at a time in the worker code. The RCv3 worker-actually-make-a-request code takes off the outer list, added by `ParallelEffects`; the RCv3 worker-orchestrate-function-that-actually-makes-a-request code believed it did the same, but did that to something that was already just the RCv3 response (so `[{}]`), leaving it with just `{}`.

Fortunately, no behavior change was needed, which is comforting given that the code apparently actually worked yesterday ;) This isn't about behavior: it's just about making the tests a little more realistic, which can be useful for inspection purposes.