nixos/tests/gitdaemon: fix spurious test failures due to flaky network

This test is sometimes flaky on hydra as at the time of the `git clone`
the network isn't really configured yet[1]. That problem doesn't seem to
occur locally but if you run it on a machine with high enough load (such
as hydra build machines). Hopefully this will make the test not flaky
anymore.

[1] https://hydra.nixos.org/build/118710378/nixlog/21/raw

nixos/tests/gitdaemon.nix

@@ -55,6 +55,9 @@ in {
     with subtest("git daemon starts"):
         server.wait_for_unit("git-daemon.service")
 
+    server.wait_for_unit("network-online.target")
+    client.wait_for_unit("network-online.target")
+
     with subtest("client can clone project.git"):
         client.succeed(
             "git clone git://server/project.git /project",