add test-end-to-end.sh iptables check

Added a quick check to ensure proper access rights for test-end-to-end.sh.

@liggitt review?
other scripts tend to do
```
... command ...
if [ $? -eq 0 ]; then
  ...
fi
```
feels a little cleaner than the string compare... not sure we get a lot by exiting with the same return code as the iptables failure