Fix test failure of extended-query-test/disable-load-balance-always.

Fix test failure of extended-query-test/disable-load-balance-always.

It expected the first SELECT to be sent to load balance node but a
preceding write query (DROP/CREATE TABLE) prevented it because it set
the writing_transaction flag. Fix is, instead of issuing DROP/CREATE
TABLE before the SELECT, issue harmless SET command after the SELECT
in extended query mode.