Golden testing assumes sbt layout when calculating rootDir for test files

[DamianReeves]

I tried using golden testing in my project that uses mill as its build system and golden testing fails because it is looking for a folder with the name "target", mill doesn't follow this structure.

It seems to me that it would be better to allow the user to configure the root directory, I had to reproduce the core of golden testing just to get things working in my project.