Test `fzf` by mocking key presses

[theogf]

This is a complicated one. To really test through, we would need to mimick a user pressing keys in the interactive picker.

Question was asked here to see if there exists a solution: https://discourse.julialang.org/t/simulating-keystrokes-to-external-program/128229

[theogf]

There can be good inspiration from Infiltrator.jl, it is using TerminalRegressionTest.jl and it seems I was missing that the prompt needs to be built on top of the emuterm cretated by the function. Will give another try.