test: add crit test

Test steps:
1) generate img files by dumping loop.sh
2) convert img files to json files
3) convert json files to img files
4) cmp original and produces img files

It also uses i/o redirection and --format nice options to
check if they work properly.

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>