crit: Speed up jenkins test ~60 times

Running crit tool 4 times per test (decode, encode, decode --pretty
and encode back again) is way too slow. The majority of time, as
it turned out, goes on python load and arguments parsing. The en-
and de-coding works pretty fast.

So doing re-code logic in one python script for ALL images is way
way faster -- ~1 hour vs ~1 minute on my box.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>