test(unikernels): Add unit test suite for Unikraft driver

[safiya2610]

Problem

The Unikraft unikernel driver in pkg/unikontainers/unikernels/unikraft.go currently lacks unit test coverage. Critical logic such as command-line parameter generation, version-based network configuration, and root filesystem handling is not protected against regressions.

Summary

This PR introduces a comprehensive table-driven unit test suite (unikraft_test.go) for the Unikraft driver.

Changes include:

• TestUnikraftCommandString: Verifies CLI parameter generation for standard executions, environment variables, network arguments, and VFS configurations (initrd).
• TestUnikraftSupportsFS & TestUnikraftSupportsBlock: Validates that Unikraft correctly reports support only for 9pfs and rejects ext2, virtiofs, and block devices.
• TestUnikraftInitVersionHandling: Verifies the version comparison logic in configureUnikraftArgs(), ensuring legacy networking arguments (netdev.ipv4_addr) are applied for Unikraft < 0.16.1, and modern arguments (netdev.ip) for >= 0.16.1.

Testing

• Tested locally via go test -v ./pkg/unikontainers/unikernels/... (All Unikraft tests pass).
• Validated that go build does not break.

Linked Issues

Fixes: #943

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[safiya2610]

[safiya2610]

@cmainas Can you please review the PR.