Previously, this test would verify a writeText file with a constant
content is at a precise store path, but this is not actually the case
and the store path has changed maybe twice since the original
"pkgs.formats: Add libconfig format generator" PR was started, the
latest time being after it was merged.
We now placehold the store path in expected.txt and substitute it just
before we run the diff, alleviating the problem.
