roachtest: fix node partition picker in c2c tests

Previously, the the partition picker would always choose src nodes to partition
because it would grab the node number relative to the destination cluster, not
relative to roachprod. This patch fixes this bug.

Epic: none

Release note: none

pkg/cmd/roachtest/tests/cluster_to_cluster.go

@@ -1735,8 +1735,9 @@ func partitionPair(ctx context.Context, c cluster.Cluster, t test.Test, srcNodes
 	srcNode := srcNodes.RandNode()[0]
 	srcTenantSQL := sqlutils.MakeSQLRunner(c.Conn(ctx, t.L(), srcNode))
 
-	var dstNode int
-	srcTenantSQL.QueryRow(t, `select split_part(consumer, '[', 1) from crdb_internal.cluster_replication_node_streams order by random() limit 1`).Scan(&dstNode)
+	var dstNodeNormalized int
+	srcTenantSQL.QueryRow(t, `select split_part(consumer, '[', 1) from crdb_internal.cluster_replication_node_streams order by random() limit 1`).Scan(&dstNodeNormalized)
+	dstNode := dstNodeNormalized + len(srcNodes)
 
 	t.L().Printf("Disconnecting Src %d, Dest %d for %.2f minutes", srcNode,
 		dstNode, disconnectDuration.Minutes())