test: fix TestHashKVWhenCompacting: ensure all goroutine finished

Signed-off-by: Benjamin Wang <benjamin.ahrtr@gmail.com>

server/mvcc/kvstore_test.go

@@ -610,10 +610,11 @@ func TestHashKVWhenCompacting(t *testing.T) {
 
 	select {
 	case <-donec:
-		wg.Wait()
 	case <-time.After(10 * time.Second):
 		testutil.FatalStack(t, "timeout")
 	}
+
+	wg.Wait()
 }
 
 // TestHashKVWithCompactedAndFutureRevisions ensures that HashKV returns a correct hash when called