test(unikraft/core): A: Add unit tests for core package

[adity1raut]

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/core package achieving 82% coverage
• All options, accessors, path helpers, and schema transforms are fully covered

Functions tested (100%)

• All UnikraftConfig accessors: Name, String, Source, Version, License,
  Compiler, CompileDate, CompiledBy, CompiledByAssoc, Type, Path
• IsUnpacked — existing dir, non-existent, empty path
• KConfig — nil map, populated map
• All CONFIG_* path helpers
• MarshalYAML — with and without kconfig
• PrintInfo, KConfigTree error path
• All options: WithVersion, WithSource, WithPath, WithKConfig,
  WithLicense, WithCompiler, WithCompileDate, WithCompiledBy, WithCompiledByAssoc
• NewUnikraftFromOptions — with UK_BASE context, option returning error
• TransformFromSchema — version string, URL source, local dir, kconfig, error paths
• Libraries — non-existent path (nil), empty lib/plat dirs
• Constants: MakeDelimeter, VerboseQuiet, VerboseBuild, VerboseExtra

Note

Libraries (43%) and KConfigTree (75%) happy paths require a real
Unikraft source tree with Config.uk files — not testable in isolation.

Related Issue

Closes #2706