Fix linting

Co-authored by: Fiona Liao <fiona.liao@grafana.com>:

Signed-off-by: Carrie Edwards <edwrdscarrie@gmail.com>

tsdb/testutil.go

@@ -14,9 +14,10 @@
 package tsdb
 
 import (
-	prom_testutil "github.com/prometheus/client_golang/prometheus/testutil"
 	"testing"
 
+	prom_testutil "github.com/prometheus/client_golang/prometheus/testutil"
+
 	"github.com/prometheus/prometheus/tsdb/chunkenc"
 
 	"github.com/stretchr/testify/require"