Make proxy test more robust

http://hydra.nixos.org/build/25322489

Changed files
+1
nixos
tests
+1
nixos/tests/proxy.nix
···
$proxy->waitForUnit("httpd");
$backend1->waitForUnit("httpd");
$backend2->waitForUnit("httpd");
+
$client->waitForUnit("network.target");
# With the back-ends up, the proxy should work.
$client->succeed("curl --fail http://proxy/");