Avoid using test.repl->N in MariaDBMonitor tests

The value may not match the nodes used by the test.

system-test/mariadbmonitor/mysqlmon_failover_stress.cpp

@@ -34,6 +34,9 @@ const time_t FAILOVER_DURATION = 5;
 // The test now runs only two failovers. Change for a longer time limit later.
 // TODO: add semisync to remove this limitation.
 
+// Number of backends
+const int N = 4;
+
 #define CMESSAGE(msg) \
     do { \
         stringstream ss; \
@@ -506,7 +509,7 @@ bool is_valid_server_id(TestConnections& test, int id)
     std::set<int> ids;
     test.repl->connect();
 
-    for (int i = 0; i < test.repl->N; i++)
+    for (int i = 0; i < N; i++)
     {
         ids.insert(test.repl->get_server_id(i));
     }