Create a test file, gzip and xz it. `gzip -tlv` will report "NOT OK" when
testing the file before and "OK" after.
```
$ dd if=/dev/random bs=1m count=1 | b64encode - > 1M.out
$ gzip 1M.out
$ xz -z 1M.out
$ gunzip -tlv ~/tmp/1M.out.xz
1089356 1416525 23.0% /home/tj/tmp/1M.out.xz
/home/tj/tmp/1M.out.xz: NOT OK
$ gunzip -tlv ~/tmp/1M.out.gz
method crc date time compressed uncompressed ratio uncompressed_name
defla 563d46f5 Jan 26 11:39 1077905 1077550 -0.1% /home/tj/tmp/1M.out
/home/tj/tmp/1M.out.gz: NOT OK
```