[homie] Enable and fix tests

[lsiepel]

The failures were caused by timing and ordering assumptions in the tests, not in production code.

• Re-enable retrieveOneAttribute, retrieveAttributes, and parseHomieTree.
• Allow sufficient time for retained MQTT messages under load.
• Wait for the channel-state callback instead of only detecting entry into a spied method.
• Wait for the MQTT subscription to complete before publishing a non-retained message, preventing publication from winning the subscription race.
• Wait for asynchronously delivered channel state before asserting it.
• Reuse and properly clean up the test scheduler instead of leaking an executor.
• Explicitly unsubscribe the temporary MQTT subscriber.
• Keep the original internal Homie subscription timeouts to avoid unnecessary delays for optional attributes.

PR was created with the assistence of codex. Manually reviewed and adapted where needed.

Fixes: #12667
Refs: #12829
Refs: #12812

[Copilot]

Pull request overview

This PR re-enables previously disabled Homie integration tests and improves their determinism by removing timing/ordering assumptions around MQTT subscription setup and asynchronous channel-state delivery.

Changes:

• Re-enabled retrieveOneAttribute, retrieveAttributes, and parseHomieTree tests and increased timeouts for MQTT message arrival under CI load.
• Adjusted assertions to wait on observable outcomes (channel-state callbacks / asynchronously delivered state) rather than relying on method-entry timing.
• Improved test resource handling by reusing a shared scheduler and explicitly unsubscribing a temporary MQTT subscriber.

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[lsiepel]

While working on these tests, codex found an issue in production code: #21417

[wborn]

This PR is initially reviewed with AI assistance.

The test synchronization and cleanup changes look good. The previously reported timing and subscription-ordering issues appear to be addressed, and no further functional issues were identified.

[wborn]

Thanks for the fixes!