postgres: add snapshot tests (#79794)

* postgres: add snapshot tests

* fixed wrong comment

Co-authored-by: Zoltán Bedi <zoltan.bedi@gmail.com>

---------

Co-authored-by: Zoltán Bedi <zoltan.bedi@gmail.com>

pkg/tsdb/grafana-postgresql-datasource/testdata/time_series/fill_value.sql

@@ -0,0 +1,15 @@
+-- SELECT $__timeGroup("time",5m,27),c,avg(v) AS "v" FROM tbl GROUP BY 1,2 ORDER BY 1,2
+-- tests fill-mode=value
+CREATE TEMPORARY TABLE tbl (
+    "time" timestamp with time zone,
+    v double precision,
+    c text
+);
+
+INSERT INTO tbl ("time", v, c) VALUES
+('2023-12-24 14:21:03 UTC', 10, 'a'),
+('2023-12-24 14:21:03 UTC', 110, 'b'),
+('2023-12-24 14:23:03 UTC', 20, 'a'),
+('2023-12-24 14:23:03 UTC', 120, 'b'),
+('2023-12-24 14:39:03 UTC', 50, 'a'),
+('2023-12-24 14:39:03 UTC', 150, 'b');