I saw that the new defag test for list was failing in CI recently, so i
reduce it's threshold from 12 to 60.
besides that, i add / improve the latency test for that other two defrag
tests (add a sensitive latency and digest / save checks)
and fix bad usage of debug populate (can't overrides existing keys).
this was the original intention, which creates higher fragmentation.
60096bc1a1