test(unikernels): add unit tests for Mewz

[r0hansaxena]

Description

Adds unit tests for the Mewz unikernel backend, covering CommandString, MonitorNetCli, MonitorBlockCli, MonitorCli, SupportsBlock, SupportsFS and Init. Tests verify the ip/gateway cmdline format, the qemu-only virtio-net device line, the no-block/no-fs guarantees, the qemu-specific monitor args (-no-reboot and isa-debug-exit) and Init's mask handling including the default /24 fallback when no mask is provided.

Related issues

• Part of Add more unit tests #96

How was this tested?

All 7 unit tests pass locally. Full package tests pass. Linter passes (make lint, 0 issues).

LLM usage

LLM assisted with locating coverage gaps only. All code was written manually and tested locally.

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.

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