test(unikernels): add unit tests for Mirage

[r0hansaxena]

Description

Adds unit tests for the Mirage unikernel backend, covering SupportsBlock, SupportsFS, MonitorNetCli, MonitorBlockCli, MonitorCli, CommandString and Init. Tests verify that hvt/spt monitors return the correct --net:service and --net-mac:service flags while other monitors return empty strings, that block passthrough uses the fixed storage ID with only the first device, and that Init correctly formats the --ipv4 address prefix and skips network setup when no mask is provided.

Related issues

• Part of Add more unit tests #96

How was this tested?

All 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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