* scheduler: stopped-yet-running allocs are still running

* scheduler: test new stopped-but-running logic

* test: assert nonoverlapping alloc behavior

Also add a simpler Wait test helper to improve line numbers and save few
lines of code.

* docs: tried my best to describe #10446

it's not concise... feedback welcome

* scheduler: fix test that allowed overlapping allocs

* devices: only free devices when ClientStatus is terminal

* test: output nicer failure message if err==nil
Co-authored-by: Mahmood Ali <mahmood@hashicorp.com>
Co-authored-by: Michael Schurter <mschurter@hashicorp.com>