test_tablets: add rack decommission test cases
Test scenarios where decommissioing a compelte rack
should succeed, and reproduce scylladb/scylladb#19475
where decommissioning a rack would fail since the
number of remaining racks is insufficient to satisfy
the replication factor, even though the number of nodes
is sufficient, enshrining this behavior.
Refs scylladb/scylladb#19475
* This PR adds unit tests and improves an error message. No backport required.
Closesscylladb/scylladb#20747
* github.com:scylladb/scylladb:
tablet_allocator: improve error message when unable to find replicas when draining
test_tablets: add rack decommission test cases
topology_experimental_raft/test_tablets: get_tablet_count_per_shard_for_host: move shards_count param to be last
test/pylib: ServerInfo: add datacenter and rack attributes
test: everywhere: drop unused imports of ServerInfo