postgres: improved tests (#84677)

* postgres: tests: better test aligning time-series results

* postgres: add snapshot test for a backward-compat code

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

@@ -0,0 +1,12 @@
+-- SELECT $__timeGroup("time",5m,27),avg(v1) AS "v1", avg(v2) AS "v2" FROM tbl GROUP BY 1 ORDER BY 1
+-- tests fill-mode=value
+CREATE TEMPORARY TABLE tbl (
+    "time" timestamp with time zone,
+    v1 double precision,
+    v2 double precision
+);
+
+INSERT INTO tbl ("time", v1, v2) VALUES
+('2023-12-24 14:21:03 UTC', 10, 110),
+('2023-12-24 14:23:03 UTC', 20, 120),
+('2023-12-24 14:39:03 UTC', 50, 150);