Add proper test for pico.abort_ddl
Test that currently exists doesnt exercise full abort ddl codepath. So currently AbortDdl raft op is never constructed in tests. This needs to be fixed.
It is not that easy to write such a test. Most probably some sort of failure injection mechanism needs to be used. There there is fail crate that may be useful but maybe there is some better way to do failure injection. To my mind fail is good enough.