postgres: add tests to handle multiple statements in the same query (#85523)

* postgres: add tests to handle multiple statements in the same query

* better test explanation

pkg/tsdb/grafana-postgresql-datasource/testdata/table/multi_stat2.sql

@@ -0,0 +1,10 @@
+-- SET TIME ZONE -7; select * from tbl; SET TIME ZONE DEFAULT; select * from tbl;
+-- we are testing that you can run multiple statements in one "query",
+-- and those statements have effects on each other, in this example
+-- we load the same database-value in different time-zones,
+-- we should receive different values.
+CREATE TEMPORARY TABLE tbl (
+    d timestamp with time zone
+);
+
+INSERT INTO tbl (d) VALUES ('2024-04-03T12:43:25UTC')