test(hypervisors): add unit tests for Firecracker

[r0hansaxena]

Description

Adds unit tests for the Firecracker VMM backend, covering BuildExecCmd and the interface methods (UsesKVM, SupportsSharedfs, Path, Ok, PreExec). Tests verify the argv layout and the JSON config written to /tmp/fc.json, memory fallback, seccomp toggle, networking, drives with rootfs flagging, initrd path priority and the vsock device.

Related issues

• Part of Add more unit tests #96

How was this tested?

All 6 unit tests pass locally. Full package tests pass. Linter passes (make lint, 0 issues).

LLM usage

LLM was used to assist with writing the tests. All code was reviewed and tested locally before submission.

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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