Avoid disk usage explosion when copying busybox

When running runc tests with temp directory with size 500M copying
busybox without preserving hardlinks causes the folder to inflate to
roughly 330M. Copying busybox twice in certain tests causes the /tmp
directory to overfill. Using `-a` preserves links which busybox uses to
implement its choice of binary to run.

Signed-off-by: Tom Godkin <tgodkin@pivotal.io>

libcontainer/integration/utils_test.go

@@ -109,7 +109,7 @@ func remove(dir string) {
 // copyBusybox copies the rootfs for a busybox container created for the test image
 // into the new directory for the specific test
 func copyBusybox(dest string) error {
-	out, err := exec.Command("sh", "-c", fmt.Sprintf("cp -R /busybox/* %s/", dest)).CombinedOutput()
+	out, err := exec.Command("sh", "-c", fmt.Sprintf("cp -a /busybox/* %s/", dest)).CombinedOutput()
 	if err != nil {
 		return fmt.Errorf("copy error %q: %q", err, out)
 	}