Fix race with task checkpoint

Because runc will delete a container after a successful checkpoint we
need to handle a NotFound error from runc on delete.

There is also a race between SIGKILL'ing the shim and it actually
exiting to unmount the tasks rootfs, we need to loop and wait for the
task to actually be reaped before trying to delete the rootfs+bundle.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

client_test.go

@@ -132,7 +132,9 @@ func TestMain(m *testing.M) {
 			}
 		}
 		if err := cmd.Wait(); err != nil {
-			fmt.Fprintln(os.Stderr, "failed to wait for containerd", err)
+			if _, ok := err.(*exec.ExitError); !ok {
+				fmt.Fprintln(os.Stderr, "failed to wait for containerd", err)
+			}
 		}
 		if err := os.RemoveAll(defaultRoot); err != nil {
 			fmt.Fprintln(os.Stderr, "failed to remove test root dir", err)