Make logging of the fawltydeps run testable without running in a subprocess

[mknorps]

When refactoring tests from using subprocess runs of fawltydeps to usage of main function we encountered some trouble with testing logger messages. As pointed out by @Nour in https://github.com/tweag/FawltyDeps/pull/271#discussion_r1163327450 we should be able to use function runs of FawltyDeps the same way we use subprocess. The contextlib function redirect_stderr did not work correctly in combination with pytest runs.

One idea is to refactor main with explicit logger or at leas stderr and capture data from there.