Provide an IFileSystem injecton to make tests more integrated

[vsdsantos]

Hi,

First of all, thanks for the excellent work on keeping the Squirrel alive 😄

Is it possible to have an IFileSystem injection on the Utility or UpdateManager classes so we can test the package in a more integrated way?

Thanks in advance

[vsdsantos]

Now I realised that I could use a Mock from the Moq library, closing this issue