vinyl: continue fixing errinj.test.lua

The range iterator open function used an obsolete indicator
for special key (NULL key), which led to selection of a wrong
range in :max() query.

The bug required a compaction process to trigger it, since
when there was only 1 range max() naturally always got the last one.
After compaction, range iterator open could yield a random
range for the special key.

Update errinj.test.lua to always trigger compaction.