fix SQLite3 race condition in test helper

Don't know why this was that way, but it was definitely wrong.
Merge branch 'jfinkhaeuser-master' (ref. #4)
make sure no-rollback test completes before connections are closed
remove pointless use of subtests
Fix error message when commit/rollback fails
Attach original exception to rollback errors
2.2.0 - Allow opting out of auto-rollback