Currently, the test assumes that when
'topology_coordinator_pause_before_processing_backlog: waiting' is
logged, the task for decommission must be there. This was based on the
assumption that topology coordinator is idle and decommission request
wakes it up. But if the server is slow enough, it may still be running
the load balancer in reaction to table creation, and block on that
injection point before decommission request was added.
Fix by waiting for the task to appear rather than the injection.
Fixes SCYLLADB-715
Both of the changed test cases stop two out of four nodes when there are
three group0 voters in the cluster. If one of the two live nodes is
a non-voter (node 1, specifically, as node 0 is the leader), a temporary
majority loss occurs, which can cause the following operations to fail.
In the case of `test_tablets_are_rebuilt_in_parallel`, the `exclude_node`
API can fail. In the case of `test_remove_is_canceled_if_there_is_node_down`,
removenode can fail with an unexpected error message:
```
"service::raft_operation_timeout_error (group
[46dd9cf1-fe21-11f0-baa0-03429f562ff5] raft operation [read_barrier] timed out)"
```
Somehow, these test cases are currently not flaky, but they become flaky in
the following commit.
We can consider backporting this commit to 2026.1 to prevent flakiness.
