Centralize handling of `uucore::error::set_exit_code()` calls.

[hanbings]

refer: #421 (comment)

This could be a method on MatcherIO, and then you wouldn't need to unset it in the tests

Note: Currently we have two uucore::error::set_exit_code() in the entire find code.

[tavianator]

This is done in e73334f