veyron/services/mgmt/node: fixing test failure on Jenkins

Replacing Mkdir() with MkdirAll() to prevent the former failing if the
test directory to be created already exists (as it is the case on
Jenkins).

Change-Id: I23a034ddb32c1a137a472287c9cee60004113c36
diff --git a/services/mgmt/node/impl/impl_test.go b/services/mgmt/node/impl/impl_test.go
index c56ebb3..79d6c2b 100644
--- a/services/mgmt/node/impl/impl_test.go
+++ b/services/mgmt/node/impl/impl_test.go
@@ -170,8 +170,8 @@
 	// This is the local filesystem location that the node manager is told
 	// to use.
 	root, perm := filepath.Join(os.TempDir(), "nodemanager"), os.FileMode(0700)
-	if err := os.Mkdir(root, perm); err != nil {
-		t.Fatalf("Mkdir(%v, %v) failed: %v", root, perm, err)
+	if err := os.MkdirAll(root, perm); err != nil {
+		t.Fatalf("MkdirAll(%v, %v) failed: %v", root, perm, err)
 	}
 	defer os.RemoveAll(root)