unit tests: Skip Windows-unrelated tests on Windows

Some of the unit tests cannot pass on Windows due to various reasons:

- fsnotify does not have a Windows implementation.
- Proxy Mode IPVS not supported on Windows.
- Seccomp not supported on Windows.
- VolumeMode=Block is not supported on Windows.
- iSCSI volumes are mounted differently on Windows, and iscsiadm is a
  Linux utility.

pkg/volume/metrics_block_linux_test.go

@@ -0,0 +1,49 @@
+//go:build !windows
+// +build !windows
+
+/*
+Copyright 2021 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package volume_test
+
+import (
+	"testing"
+
+	. "k8s.io/kubernetes/pkg/volume"
+	volumetest "k8s.io/kubernetes/pkg/volume/testing"
+)
+
+func TestGetMetricsBlockInvalid(t *testing.T) {
+	// TODO: Enable this on Windows once support VolumeMode=Block is added.
+	metrics := NewMetricsBlock("")
+	actual, err := metrics.GetMetrics()
+	expected := &Metrics{}
+	if !volumetest.MetricsEqualIgnoreTimestamp(actual, expected) {
+		t.Errorf("Expected empty Metrics from uninitialized MetricsBlock, actual %v", *actual)
+	}
+	if err == nil {
+		t.Errorf("Expected error when calling GetMetrics on uninitialized MetricsBlock, actual nil")
+	}
+
+	metrics = NewMetricsBlock("/nonexistent/device/node")
+	actual, err = metrics.GetMetrics()
+	if !volumetest.MetricsEqualIgnoreTimestamp(actual, expected) {
+		t.Errorf("Expected empty Metrics from incorrectly initialized MetricsBlock, actual %v", *actual)
+	}
+	if err == nil {
+		t.Errorf("Expected error when calling GetMetrics on incorrectly initialized MetricsBlock, actual nil")
+	}
+}