Test suite: avoid writing to script while it may be executing
When "the network is plugged" the NetworkManager hooks are executed, so when we in the other step starts writing to one of those hooks, 00-firewall.sh, it sometimes happens that the write fails with: [Errno 26] Text file busy (refs: #19736 (closed)). We can avoid that by just flipping the order of these two steps, so the writing is done before the NetworkManager hooks are run.
Closes #19736 (closed)