postgres: tests: improved number->timestamp tests (#81495)

* postgres: tests: nicer values

* postgres: tests: add float-to-timestamp tests

pkg/tsdb/grafana-postgresql-datasource/testdata/table/timestamp_convert_integer.sql

@@ -8,5 +8,5 @@ CREATE TEMPORARY TABLE tbl (
 );
 
 INSERT INTO tbl (reallyt, "time", n, timeend) VALUES
-('2023-12-24T14:21:27 UTC', 1703161287, 1703161287, 1703161312),
+('2023-12-21T12:21:27 UTC', 1703161287, 1703161287, 1703161312),
 (NULL, NULL, NULL, NULL);