[PATCH] hs_standby_disallowed test fix

Hello.

There is a recent commit about changes in way read-only commands are
prevented to be executed [1].

It seems like hs_standby_disallowed test is broken now.

So, a simple patch to fix the test is attached.

Thanks,
Michail.

[1]
https://www.postgresql.org/message-id/flat/154701965766.11631.2240747476287499810.pgcf%40coridan.postgresql.org#168075c6e89267e11b862aa0c55b1910