test: allow slowdown when race detector is enabled

benchmarks_test.go

@@ -50,7 +50,7 @@ func TestSmallPackets(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if slowdown > 0.15 {
+	if slowdown > 0.15 && !raceEnabled {
 		t.Fatalf("Slowdown from mplex was >15%%: %f", slowdown)
 	}
 }

norace_test.go

+//+build !race
+
+package multiplex
+
+var raceEnabled = false

race_test.go

+//+build race
+
+package multiplex
+
+var raceEnabled = true