gnu: containerd: Fix test failure with Go 1.13.

* gnu/packages/docker.scm (containerd)[source]: Add patch.
* gnu/packages/patches/containerd-test-with-go1.13.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add patch.

gnu/packages/patches/containerd-test-with-go1.13.patch

@@ -0,0 +1,21 @@
+Compatibility fix for go 1.13, flag.Parse() shouldn't be called during
+package initialization.
+https://golang.org/doc/go1.13#testing
+--- a/client_test.go	2020-02-12 14:50:28.991245371 -0500
++++ b/client_test.go	2020-02-12 15:12:37.383523980 -0500
+@@ -49,7 +49,6 @@
+ 	flag.StringVar(&address, "address", defaultAddress, "The address to the containerd socket for use in the tests")
+ 	flag.BoolVar(&noDaemon, "no-daemon", false, "Do not start a dedicated daemon for the tests")
+ 	flag.BoolVar(&noCriu, "no-criu", false, "Do not run the checkpoint tests")
+-	flag.Parse()
+ }
+ 
+ func testContext() (context.Context, context.CancelFunc) {
+@@ -59,6 +58,7 @@
+ }
+ 
+ func TestMain(m *testing.M) {
++	flag.Parse()
+ 	if testing.Short() {
+ 		os.Exit(m.Run())
+ 	}