nixosTests.ngingx-http3: fix race condition

make the client wait for its network to come up before starting test

without the wait, the client might try to access the network before its
own network stack is up, as evidenced in
https://hydra.nixos.org/build/299048133/nixlog/9/tail

(cherry picked from commit 962db80796)

nixos/tests/nginx-http3.nix

@@ -90,6 +90,7 @@ builtins.listToAttrs (
 
           server.wait_for_unit("nginx")
           server.wait_for_open_port(443)
+          client.wait_for_unit("network-online.target")
 
           # Check http connections
           client.succeed("curl --verbose --http3-only https://acme.test | grep 'Hello World!'")