This test exercises the behavior of clients that become disconnected
and have their allocations replaced. Future test cases will exercise
the `max_client_disconnect` field on the job spec.
The CLI helpers in the rescheduling test were intended for shared use, but
until some other tests were written we didn't want to waste time making them
generic. This changeset refactors them and adds some new helpers associated
with the node drain tests (under separate PR).
Extends the BasicAllocStats test to include a test for Windows
clients, exercising stats via a powershell `raw_exec` job.
Adds `ListLinuxClientNodes` and `ListWindowsClientNodes` utils so that
we can scope tests to run only when Linux or Windows clients are
available. This prevents waiting on timeouts when running a subset of
the tests against a development cluster (vs our nightly test
cluster).
