MXS-3410 Drop anonymous users also in Xpand

As Xpand does not have a mysql.user table but a system.users
table, we need to arrange things so that the query used for
selecting annymous users can be different depending on the cluster.

system-test/maxtest/include/maxtest/mariadb_nodes.hh

@@ -535,8 +535,14 @@ public:
      */
     std::string cnf_server_name;
 
-private:
+protected:
+    /**
+     * @returns SELECT that returns anonymous users in such a way that each returned row
+     *          can directly be given as argument to DROP USER.
+     */
+    virtual std::string anonymous_users_query() const;
 
+private:
     bool check_master_node(MYSQL* conn);
     bool bad_slave_thread_status(MYSQL* conn, const char* field, int node);
 };

system-test/maxtest/include/maxtest/xpand_nodes.hh

@@ -59,4 +59,7 @@ public:
 
     std::string block_command(int node) const override;
     std::string unblock_command(int node) const override;
+
+private:
+    std::string anonymous_users_query() const override;
 };

system-test/maxtest/src/mariadb_nodes.cc

@@ -1028,6 +1028,11 @@ std::vector<int> Mariadb_nodes::get_all_server_ids()
     return rval;
 }
 
+std::string Mariadb_nodes::anonymous_users_query() const
+{
+    return "SELECT CONCAT('\\'', user, '\\'@\\'', host, '\\'') FROM mysql.user WHERE user = ''";
+}
+
 bool Mariadb_nodes::prepare_for_test(MYSQL* conn)
 {
     int local_result = 0;
@@ -1047,9 +1052,7 @@ bool Mariadb_nodes::prepare_for_test(MYSQL* conn)
         local_result++;
     }
 
-    if (mysql_query(conn,
-                    "SELECT CONCAT('\\'', user, '\\'@\\'', host, '\\'') FROM mysql.user WHERE user = ''")
-        == 0)
+    if (mysql_query(conn, anonymous_users_query().c_str()) == 0)
     {
         MYSQL_RES* res = mysql_store_result(conn);
 

system-test/maxtest/src/xpand_nodes.cpp

@@ -275,3 +275,8 @@ std::string Xpand_nodes::unblock_command(int node) const
 
     return command;
 }
+
+std::string Xpand_nodes::anonymous_users_query() const
+{
+    return "SELECT CONCAT('\\'', user, '\\'@\\'', host, '\\'') FROM system.users WHERE user = ''";
+}