Running \Drupal\KernelTests\Config\DefaultConfigTest creates a directory called : in DRUPAL_ROOT

Problem/Motivation

Running \Drupal\KernelTests\Config\DefaultConfigTest creates a directory called : in DRUPAL_ROOT. This artefact shouldn't exist.

It gets created by image_install. It happens because file_default_scheme() depends on system config BUT file.inc is part of core.

Steps to reproduce:

1. Run Drupal\KernelTests\Config\DefaultConfigTest locally
2. Check for the existence of a directory called : in the root of the checkout. It will contain another directory called styles

Proposed resolution

The simplest fix here is to make it return 'public' if the configuration does not exist. Once it does then the directory is created as expected in virtual file system set up by KernelTestBase.

Remaining tasks

User interface changes

None

API changes

None.

Data model changes

None