sync/test: skip test ConcurrentTruncate on uptobox (take 2)

The test is not applicable to uptobox which can't upload empty files.
The test was not skipped as intended because the direct error was compared.
This fix will compare error Cause because Sync wraps the error.

fs/sync/sync_test.go

@@ -2091,7 +2091,7 @@ func testSyncConcurrent(t *testing.T, subtest string) {
 	fstest.CheckItems(t, r.Fremote, itemsBefore...)
 	stats.ResetErrors()
 	err := Sync(ctx, r.Fremote, r.Flocal, false)
-	if err == fs.ErrorCantUploadEmptyFiles {
+	if errors.Cause(err) == fs.ErrorCantUploadEmptyFiles {
 		t.Skipf("Skip test because remote cannot upload empty files")
 	}
 	assert.NoError(t, err, "Sync must not return a error")