привет мир  (Add testcase for unicode queries)

I noticed that the IsString instance for Identifier and Query look suspect
from an efficiency standpoint.   I briefly considered using newtype deriving
instead,  but then remembered that this would probably break anybody using
unicode inside queries.

While this wouldn't affect me personally,  postgresql-simple does have a
number of international users and contributors.   So I added this to the
test suite,  in case I do decide to try to tweak the IsString instance
at some point.