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_stat1.sql

@@ -0,0 +1,7 @@
+-- SELECT t FROM tbl WHERE t='one'; SELECT t FROM tbl WHERE t='three';
+-- lib/pq allows multiple queries to run, and return results for all the queries
+CREATE TEMPORARY TABLE tbl (
+    t text
+);
+
+INSERT INTO tbl (t) VALUES ('one'),('two'),('three');