Fix health checking for ephemeral poststart tasks

[beautifulentropy]

This PR updates the logic in the Nomad client's alloc health tracker which
erroneously marks existing healthy allocations with dead poststart ephemeral
tasks as unhealthy even if they were already successful during a previous
deployment. For repro see: #10058 (comment)

Currently, users are having to insert sleep after short-lived ephemeral tasks.
This change ensures that poststart lifecycle tasks which have succeeded,
even before 'min_healthy_time', will not result in the whole allocation being
marked as unhealthy.

I've attempted to include test coverage that sticks to your existing conventions.

• Tracker will not attempt to evaluate the health of poststart ephemeral tasks
  which have already succeeded with any running duration.
• Add a mock and helper for deployments of allocations with poststart tasks

Fixes #9254
Fixes #10058

[hashicorp-cla]

All committers have signed the CLA.

[tgross]

LGTM! Thanks @beautifulentropy!

[tgross]

This will ship in the next major release (1.3.0) and get backported to 1.2.x and 1.1.x. Thanks again!

[github-actions]

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