test(unikraft/plat): Add unit tests for plat package

[adity1raut]

…overage

Prerequisite checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes locally.

Description of changes

Summary

• Adds unit tests for unikraft/plat package achieving 100% coverage

Functions tested

• All PlatformConfig accessors: Name, String, Source, Version, Type, Path
• IsUnpacked — existing dir, non-existent, empty path
• KConfigTree — returns nil, nil
• MarshalYAML — returns nil, nil
• PrintInfo — returns non-empty string
• KConfig — correct CONFIG_PLAT_* keys for fc, firecracker, kraftcloud,
  kvm, qemu, xen, linuxu, unknown
• TransformFromSchema — string input, alias rewrite (firecracker→fc,
  kvm→qemu), unknown name kept as-is, non-string errors, UK_BASE context
• All options: WithName, WithVersion, WithSource, WithPath,
  WithInternal, WithKConfig

Related Issue

Closes #2708

Test plan

• Run go test kraftkit.sh/unikraft/plat -cover → 100%
• Run golangci-lint run ./unikraft/plat/... → 0 issues