test: adjust `schema_change_test` to include new `system.raft_config` table
test: adjust `schema_change_test` to include new `system.raft_config` table

Check that the new table uses null sharder.

Tests: unit(dev)

Signed-off-by: Pavel Solodovnikov <2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>

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