ITS#7841 trivial cleanup

TRIVIA - lmdb: clean testdb-dir while "make test".
ITS#7959 Check fstat return value

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>
ITS#7961 fix txn init

More fallout from 4d02c741b120786df1b87ee9ed49c1d3f9bc7522