This is necessary to show any RootFs usage on systems where the backing
filesystem of overlay2 is xfs.

The current test only created directories (for mount points) in the
upper layer of the overlay. Outside of the mount namespace, only the
directories are visible. When running `du` on those, usually filesystems
will show some usage, but not xfs, which shows a disk usage of 0 for
directories.

Fix this by creating a file in the root directory, outside the volumes,
in order to trigger some disk usage that can be measured by `du`.

Tested: make test-e2e-node REMOTE=true HOSTS=centos-e2e-node FOCUS="Summary API"