MySQL: snapshot tests (#84846)

mysql: add snapshot tests

pkg/tsdb/mysql/testdata/time_series/fill_null.sql

@@ -0,0 +1,15 @@
+-- SELECT $__timeGroupAlias("time",5m,NULL),c,avg(v) AS "v" FROM tbl GROUP BY 1,2 ORDER BY 1,2
+-- tests fill-mode=null
+CREATE TABLE tbl (
+    `time` timestamp NOT NULL,
+    v double precision,
+    c text
+);
+
+INSERT INTO tbl (`time`, v, c) VALUES
+('2023-12-24 14:21:03+00:00', 10, 'a'),
+('2023-12-24 14:21:03+00:00', 110, 'b'),
+('2023-12-24 14:23:03+00:00', 20, 'a'),
+('2023-12-24 14:23:03+00:00', 120, 'b'),
+('2023-12-24 14:39:03+00:00', 50, 'a'),
+('2023-12-24 14:39:03+00:00', 150, 'b');