ut: skip TestStartNetworkMonitor for non-root

It requires root to manipulate netns and otherwise fails
like below:

=== RUN   TestStartNetworkMonitor
--- FAIL: TestStartNetworkMonitor (0.00s)
        Error Trace:    sandbox_test.go:1481
        Error:          Expected nil, but got: &errors.errorString{s:"Error switching to ns /proc/6648/task/6651/ns/net: operation not permitted"}

Signed-off-by: Peng Tao <[email protected]>

virtcontainers/sandbox_test.go

@@ -1458,6 +1458,9 @@ func TestGetNetNs(t *testing.T) {
 }
 
 func TestStartNetworkMonitor(t *testing.T) {
+	if os.Getuid() != 0 {
+		t.Skip("Test disabled as requires root user")
+	}
 	trueBinPath, err := exec.LookPath("true")
 	assert.Nil(t, err)
 	assert.NotEmpty(t, trueBinPath)