test: modernize test-cluster-master-error

Some stylistic changes to bring this more in line with what our
tests currently look like, and add a note about general flakiness.

PR-URL: https://github.com/nodejs/node/pull/34685


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ricky Zhou <0x19951125@gmail.com>