libguestfs.tests: Add more testcases for virt-edit options.

Add three testcase:
1. test for format option
2. test for backup option
3. test for connect_uri option
```
Usage:
  virt-edit [--options] -d domname file [file ...]
  virt-edit [--options] -a disk.img [-a disk.img ...] file [file ...]
Options:
  -a|--add image       Add image
  -b|--backup .ext     Backup original as original.ext
  -c|--connect uri     Specify libvirt URI for -d option
  -d|--domain guest    Add disks from libvirt guest
  --echo-keys          Don't turn off echo for passphrases
  -e|--expr expr       Non-interactive editing using Perl expr
  --format[=raw|..]    Force disk format for -a option
  --help               Display brief help
  --keys-from-stdin    Read passphrases from stdin
  -v|--verbose         Verbose messages
  -V|--version         Display version and exit
  -x                   Trace libguestfs API calls
```

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>

Rely on:
https://github.com/autotest/virt-test/pull/1596