This required adding a new ContextualDefaultOption class
to get a default callable that also receives the context.
Moreover, a but was fixed in the function that prompts
for the prepend/append_text, as if the file was not touched/modified,
instead of reusing the text provided in input, an empty text was used instead.

Sets prepend/append text when setting up the computer/code for the non-interactive tests.
The correct duplication was already tested by an assertion, but the prepend and append texts were set to the default value of an empty string.