Add unit tests

[HardCodeDev777]

Currently only for Core/Utils logic, not for Commands(since it's only rendering and executing Core methods).

Most of the methods are easy to test. But some might be tricky like GetRepositoriesNamesFromSetup, because it requires real repository:

foreach(var folder in Directory.EnumerateDirectories(setup.RepositoriesFolder))
{
    if (Repository.IsValid(folder)) repositories.Add(Path.GetFileName(folder));
}