Regression tests for fetcher-based derivations

[sternenseemann]

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By supplying an URL to e.g. an archive or git repository, cabal2nix can generate expressions using fetcher functions from nixpkgs. We have no regression tests for this, as it requires running a nix-prefetch-* script, unfortunately, which lead to this problem not being caught. Not sure how we can make a comprehensive test suite for this.