postgres: support running multiple postgres integration tests

Makes it possible to run mysql integration tests for both
grafana config db and tsdb at the same time using
GRAFANA_TEST_DB=postgres go test ./pkg/...

docker/blocks/postgres_tests/setup.sql

@@ -0,0 +1,3 @@
+CREATE DATABASE grafanadstest;
+REVOKE CONNECT ON DATABASE grafanadstest FROM PUBLIC;
+GRANT CONNECT ON DATABASE grafanadstest TO grafanatest;