path.TestWalk: disable error case if root

(chmod 0 doesn't cause errors for root)

Fixes #22.

R=gri
https://golang.org/cl/152120

src/pkg/path/path_test.go

@@ -233,6 +233,7 @@ func TestWalk(t *testing.T) {
 	}
 	checkMarks(t);
 
+	if os.Getuid() != 0 {
 		// introduce 2 errors: chmod top-level directories to 0
 		os.Chmod(Join(tree.name, tree.entries[1].name), 0);
 		os.Chmod(Join(tree.name, tree.entries[3].name), 0);
@@ -258,6 +259,7 @@ func TestWalk(t *testing.T) {
 		}
 		// the inaccessible subtrees were marked manually
 		checkMarks(t);
+	}
 
 	// cleanup
 	os.Chmod(Join(tree.name, tree.entries[1].name), 0770);