The "$source"/var/lib/snapd/seed/systems directory only exists in certain
scenarios related to TPM-backed FDE. When the directory does not exist,
attempting to bind-mount it to /var/lib/snapd/seed/systems crashes the
install with a CalledProcessError.
We now make sure the directory exists before trying to mount it. For
dry-run test cases, we added a configuration item that simulates the
presence (or the non-presence) of the systems directory on the source.
Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>