fix racy tests when editing xl.getDisks (#8879)

cmd/xl-v1-healing_test.go

@@ -270,6 +270,7 @@ func TestHealObjectXL(t *testing.T) {
 	}
 
 	xlDisks := xl.getDisks()
+	z.zones[0].xlDisksMu.Lock()
 	xl.getDisks = func() []StorageAPI {
 		// Nil more than half the disks, to remove write quorum.
 		for i := 0; i <= len(xlDisks)/2; i++ {
@@ -277,6 +278,7 @@ func TestHealObjectXL(t *testing.T) {
 		}
 		return xlDisks
 	}
+	z.zones[0].xlDisksMu.Unlock()
 
 	// Try healing now, expect to receive errDiskNotFound.
 	_, err = obj.HealObject(context.Background(), bucket, object, false, false, madmin.HealDeepScan)