Line numbers in diffs are off by the number of snapshot header fields

[skuzzle]

Our rendered diffs contain line numbers that are based on the raw test output. Snapshot files however contain some extra header lines at the beginning of the file. Thus, when looking up changed lines in the actual snapshot file, the numbers reported in the diff are off by the number of header lines in the snapshot file.

I think we should adjust the reported line numbers by the amount of header lines so that it is easy to find the respective lines in the persisted snapshot file