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/...

pkg/services/sqlstore/sqlstore.go

@@ -278,3 +278,10 @@ func IsTestDbMySql() bool {
 	return false
 }
 
+func IsTestDbPostgres() bool {
+	if db, present := os.LookupEnv("GRAFANA_TEST_DB"); present {
+		return db == dbPostgres
+	}
+
+	return false
+}