nixos/nextcloud: add test verifying that bind-mounts and special places of /var/lib/nextcloud are OK
This is a regular source for confusion and had no test-coverage so far,
so adding two tests that
* Make sure Nextcloud running in a different location (tmpfs mount here as mock for the mount
of a different disk).
* A bind-mount of Nextcloud from a disk-mount to /var/lib/nextcloud can
be created.