postgres: improve snapshot-tests for long/wide situations (#84387)

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

@@ -0,0 +1,10 @@
+-- SELECT * FROM tbl WHERE false
+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 14:30:03 UTC', 10, 110),
+('2023-12-24 14:31:03 UTC', 20, 120);