Can't open a transaction within an on-commit hook on SQLite.

[carljm]

This is because opening a transaction on SQLite in autocommit-off raises an error, but we don't have a way to hook into the point when autocommit is turned back on (atomic doesn't call set_autocommit(True) for SQLite, it just manually sets connection.autocommit = True).

This test is currently marked xfail for SQLite.