Fix race in test

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

middleware/markdown/watcher_test.go

@@ -3,6 +3,7 @@ package markdown
 import (
 	"fmt"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 )
@@ -23,12 +24,18 @@ func TestWatcher(t *testing.T) {
 	}
 	out = ""
 	i = 0
+	var mu sync.Mutex
 	stopChan = TickerFunc(interval, func() {
 		i++
+		mu.Lock()
 		out += fmt.Sprint(i)
+		mu.Unlock()
 	})
 	time.Sleep(interval * 10)
-	if !strings.HasPrefix(out, expected) || out == expected {
+	mu.Lock()
+	res := out
+	mu.Unlock()
+	if !strings.HasPrefix(res, expected) || res == expected {
 		t.Fatalf("expected (%v) must be a proper prefix of out(%v).", expected, out)
 	}
 }