This means that configuration backend tests no longer use stubs and
(mostly) avoid internal dependencies in the tests. Instead of stubs and
mock-fs, the tests use [testfs](https://github.com/balena-io-modules/mocha-pod#working-with-the-filesystem)
which allows working with a real filesystem and ensuring everything is
re-set between tests.
This is the last change needed in order to be able to merge #1971. Here is the list of changes
- [x] Migrate splash image backend tests
- [x] Migrate extlinux backend tests
- [x] Migrate config.txt backend tests
- [x] Migrate extra-uenv config tests
- [x] Migrate odmdata config tests
- [x] Migrate config utils tests
- [x] Migrate device-config tests
Change-type: patch
