Fix test build failures with GCC 8

GCC 8 enables -Werror=return-type by default which combined with -Werror
causes the build to fail.

maxscale-system-test/blob_test.cpp

@@ -188,6 +188,8 @@ int check_longblob_data(TestConnections* Test, MYSQL * conn, unsigned long chunk
     mysql_stmt_free_result(stmt);
 
     mysql_stmt_close(stmt);
+
+    return 0;
 }