Fix: empty diffs for 'execution_output.txt'

This fixes empty diffs for preprocessed files by setting the option `source` for the third parameter when initializing `Diffy::Diff`.

If the `FileTreeSpecContext` is configured with `has_special_handling_for` for an expected file, and this method, doesn't return a `Pathname` but instead a `String` then the strings have not been properly compared. This caused that an empty but erroneous diff was shown.