Add printf attribute to all variadic functions

The test methods that take printf style input now have the printf
attribute. This enables format checks making oversights less likely.

Also fixed any existing errors in the code. Only the one in
test_binlog_fnc.cpp would've had an actual effect.

maxscale-system-test/mariadb_func.h

@@ -26,6 +26,8 @@
 #include <string>
 #include <vector>
 
+#include <maxbase/ccdefs.hh>
+
 typedef std::vector<std::string> Row;
 typedef std::vector<Row> Result;
 
@@ -119,7 +121,7 @@ static MYSQL* open_conn_no_db(int port, std::string ip, std::string user, std::s
  * @param ...       Parameters for @c format
  * @return 0 in case of success
  */
-int execute_query(MYSQL* conn, const char* format, ...);
+int execute_query(MYSQL* conn, const char* format, ...) mxb_attribute((format(printf, 2, 3)));
 
 /**
  * @brief execute_query_from_file Read a line from a file, trim leading and trailing whitespace and execute it.