query_unittest: Fix argument to set()

Commit e431074f introduced an uncatched bug. This patch fixes this. The
set is expecting a list or iteratable to work on, so it splitted the
provided instance name into a set of characters. This caused the
exp_status never been set and therefore not catched in one assert rule
further below who checks that every status was tested.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>