test: drop TODO points and emergency rollbacks from pipeline tests

test: drop TODO points and emergency rollbacks from pipeline tests

The previous changeset restored some sanity. The behaviour in
`test_errors_raised_on_commit` is not inconsistent with the non-pipeline
mode because the two are not comparable: in non-pipeline mode the
exception is raised by `execute()`, not by `commit()`. If an exception
is raised in `commit()` itself, it is already handled consistently, as
`test_error_on_commit` shows.