mysql: mysql tests should use a db server with UTC

To get rid of issues involving date/time when testing.
Also, makes it possible to run mysql integration tests for both
grafana config db and tsdb at the same time using
GRAFANA_TEST_DB=mysql go test ./pkg/...

docker/blocks/mysql_tests/setup.sql

@@ -0,0 +1,2 @@
+CREATE DATABASE grafana_ds_tests;
+GRANT ALL PRIVILEGES ON grafana_ds_tests.* TO 'grafana';