test: capture the QEMU client serial console on failure

[pasinskim]

The failure teardown only prints device output over SSH, which is empty when the device is unreachable -- which is exactly the "Device never rebooted" failure mode (a hung guest reset). Capture the client's serial console via "docker compose logs" (which works without SSH) into the per-test logs, so the guest's actual reboot behaviour (e.g. "reboot: machine restart" followed by silence) is visible directly in CI artifacts.

Changelog: None

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Build Configuration Matrix

Key	Value
INTEGRATION_REV	pull/2943/head
RUN_TESTS_FULL_INTEGRATION	true