[Testing]: Add unit test coverage for Unikraft version parsing and boot commandline generation

[ParthMozarkar]

Summary

pkg/unikontainers/unikernels/unikraft.go is the core handler for Unikraft, the primary unikernel OS supported by urunc. It manages boot argument construction based on Unikraft versioning (< 0.16.1 legacy compatibility vs. modern vfs.fstab, netdev.ip, env.vars).

While sibling files mirage_test.go and mewz_test.go exist, unikraft.go currently has zero unit tests. Any regression in version parsing or command-line generation could break Unikraft container deployments in Kubernetes/containerd.

Expected Behavior

pkg/unikontainers/unikernels should include a unikraft_test.go test suite that verifies:

1. Version fallback logic (ErrUndefinedVersion, ErrVersionParsing, version >= 0.16.1 vs < 0.16.1).
2. Boot command line generation via CommandString() (environment variables env.vars=[ ... ], console string, network IP string, rootfs vfs.fstab).
3. File system support (SupportsFS("9pfs")) and block support (SupportsBlock()).

Proposed Changes

• Create pkg/unikontainers/unikernels/unikraft_test.go with table-driven tests covering modern/legacy Unikraft versions, architecture-specific console settings (arm64 vs amd64), and error cases.

[Nachiket-Roy]

#96 already tracks unit test coverage, so you can link your PR with it instead of separate tracking. Thanks!