Jl/add unit testing

[jordlay]

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I've added UTs for the things that could be done easily without worrying about interacting with Azure. That means the generate config and build commands.

They currently just check that the code doesn't hit any errors and that something is output. We could later validate that the output is sensible, but that is a lot more effort and a job for another day.

In addition:

• I've tweaked the code so that file paths in the input files are relative to that input file (I think that is the right thing to do, but it also made writing UTs more intuitive).
• Fixed up some linting

Don't worry about reviewing the files in the mock_* folders, they are just example Helm charts and other artifacts.

[Cyclam]

A few comments, but otherwise looks good. Thanks :-)