Add test for snapshot current and parent test

```
Signed-off-by: Kyla Zhang <weizhan@redhat.com>

  NAME
    snapshot-current - Get or set the current snapshot

  SYNOPSIS
    snapshot-current <domain> [--name] [--security-info] [<snapshotname>]

  DESCRIPTION
    Get or set the current snapshot

  OPTIONS
    [--domain] <string>  domain name, id or uuid
    --name           list the name, rather than the full xml
    --security-info  include security sensitive information in XML dump
    [--snapshotname] <string>  name of existing snapshot to make current


  NAME
    snapshot-parent - Get the name of the parent of a snapshot

  SYNOPSIS
    snapshot-parent <domain> [<snapshotname>] [--current]

  DESCRIPTION
    Extract the snapshot's parent, if any

  OPTIONS
    [--domain] <string>  domain name, id or uuid
    [--snapshotname] <string>  find parent of snapshot name
    --current        find parent of current snapshot
```