DEV(cache_critical_dns): improve postgres_healthcheck

The `PG::Connection#ping` method is only reliable for checking if the
given host is accepting connections, and not if the authentication
details are valid.

This extends the healthcheck to confirm that the auth details are
able to both create a connection and execute queries against the
database.

We expect the empty query to return an empty result set, so we can
assert on that. If a failure occurs for any reason, the healthcheck will
return false.

script/cache_critical_dns

@@ -158,16 +158,18 @@ ensure
 end
 
 def postgres_healthcheck(host:, user:, password:, dbname:)
-  response = PG::Connection.ping(
+  client = PG::Connection.new(
     host: host,
     user: user,
     password: password,
     dbname: dbname,
     connect_timeout: 2,  # minimum
   )
-  response == PG::Constants::PQPING_OK
+  client.exec(';').none?
 rescue
   false
+ensure
+  client.close if client
 end
 
 HEALTH_CHECKS = {