This diff attempts to modify the errors reported by our custom resolver by matching more strings from the stdlib. Part of https://github.com/ooni/probe/issues/1733 and diff has been extracted from https://github.com/ooni/probe-cli/pull/506.
While there, modernize the way in which we run tests to avoid depending on the fake files scattered around the tree and to use some well defined mock structures instead. Part of https://github.com/ooni/probe/issues/1591