kubelet/userns: Use t.TempDir()

These tests will create the userns record mapping file, so let's use a
temporal directory for that.

Without specifying one, by mistake we were using the
"/tmp/non-existant-dir.This-is-not-used-in-tests/" directory.

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>

pkg/kubelet/userns/userns_manager_test.go

@@ -297,6 +297,7 @@ func TestCleanupOrphanedPodUsernsAllocations(t *testing.T) {
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {
 			testUserNsPodsManager := &testUserNsPodsManager{
+				podDir:  t.TempDir(),
 				podList: tc.listPods,
 			}
 			m, err := MakeUserNsManager(testUserNsPodsManager)