Update integration test to support windows

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

client_unix_test.go

@@ -0,0 +1,13 @@
+// +build !windows
+
+package containerd
+
+const (
+	defaultRoot    = "/var/lib/containerd-test"
+	defaultAddress = "/run/containerd-test/containerd.sock"
+	testImage      = "docker.io/library/alpine:latest"
+)
+
+func platformTestSetup(client *Client) error {
+	return nil
+}