refactor scheduler tests for node down/disconnected

While working on #20462 #12319 I found that some of our scheduler tests around
down nodes or disconnected clients were enforcing invariants that were
unclear. This changeset pulls out some minor refactorings so that the bug fix PR
is easier to review. This includes:

* Migrating a few tests from `testify` to `shoenig/test` that I'm going to touch
  in #12319 anyways.
* Adding test names to the node down test
* Update the disconnected client test so that we always re-process the
  pending/blocked eval it creates; this eliminates 2 redundant sub-tests.
* Update the disconnected client test assertions so that they're explicit in the
  test setup rather than implied by whether we re-process the pending/blocked
  eval.

Ref: #20462
Ref: #12319