test: raft: regression test for storing cluster configuration when taking snapshots
test: raft: regression test for storing cluster configuration when taking snapshots

Before the fix introduced in the previous patch, the cluster would
forget its configuration when taking a snapshot, making it unable to
reelect a leader. This regression test catches that.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla