Take -Wall into use for tests

Take almost all warnings into use except `format-overflow` and
`unused-function`. format-overflow needs to be handled in a separate
commit.

maxscale-system-test/mysqlmon_failover_rolling_master.cpp

@@ -185,7 +185,6 @@ void check(XTestConnections& test)
     }
 
     std::string values;
-    MYSQL_ROW row;
     int num_rows = mysql_num_rows(pRes);
     test.add_result(num_rows != i_end, "Query returned %d rows when %d rows were expected",
                     num_rows, i_end);