Add few test cases for nbd+unix style URIs with few corner cases.

The NBD URI syntax is documented at
https://github.com/NetworkBlockDevice/nbd/blob/master/doc/uri.mdSigned-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>