The fault `twophase_transaction_commit_prepared` would cause the segment to
PANIC while writing a `COMMIT PREPARED` record on the segment. The master would
then retry the `COMMIT PREPARED` while the postmaster was still resetting on the
segment causing the master itself to `PANIC`. This patch introduces a new fault
injector `finish_prepared_before_commit`  which will not cause a `PANIC`, but
just error out since the intent of the test is to ensure that the retry works
correctly.