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/table/time_group_compat_case2.sql

@@ -0,0 +1,10 @@
+-- SELECT $__timeGroup("t",5m,NULL) ,avg(v) FROM tbl GROUP BY 1
+-- we change behavior based on what follows the timegroup, see:
+-- https://github.com/grafana/grafana/blob/0b4c81158ea32a671e7788b888b086082343407c/pkg/tsdb/grafana-postgresql-datasource/macros.go#L36
+CREATE TEMPORARY TABLE tbl (
+    t timestamp with time zone,
+    v double precision
+);
+
+INSERT INTO tbl (t, v) VALUES
+('2023-12-24 14:30:03 UTC', 42);