There are 4 barrier-only stages when migrating a tablet and the test needs to fail pending/leaving replica that handles it in order to validate how coordinator handles dead node. Failing the barrier is done by suspending it with injection code and stopping the node without waking it up. The main difficulty here is how to tell one barrier RPC call from another, because they don't have anything onboard that could tell which stage the barrier is run for. This PR suggests that barrier injection code looks directly into the system.tablets table for the transition stage, the stage is already there by the time barrier is about to ack itself over RPC.
refs: #16527Closesscylladb/scylladb#17450
* github.com:scylladb/scylladb:
topology.tablets_migration: Handle failed use_new
topology.tablets_migration: Handle failed write_both_read_new
topology.tablets_migration: Handle failed write_both_read_old
topology.tablets_migration: Handle failed allow_write_both_read_old
test/tablets_migration: Add conditional break-point into barrier handler
replica: Add helper to read tablet transition stage
topology_coordinator: Add action_failed() helper
ec6ed18b5c