Speed up buildbot by requiring --big-test for some slow tests

mysql-test/t/mysql_client_test_nonblock.test

@@ -2,6 +2,8 @@
 
 # The non-blocking API is not supported in the embedded server.
 -- source include/not_embedded.inc
+# This test is slow on buildbot.
+--source include/big_test.inc
 
 SET @old_general_log= @@global.general_log;
 SET @old_slow_query_log= @@global.slow_query_log;