nixos/tests/gitdaemon: deflake by using systemd-tmpfiles

git-daemon won't start up if its project directory (here /git) doesn't
exist. If we try to create it using the test harness, then we're racing
whether we manage to connect to the backdoor vs. the startup speed of
git-daemon.

Instead, use systemd-tmpfiles, which is guaranteed(?) to run before
network.target and thus before git-daemon.service starts.

nixos/tests/gitdaemon.nix

···
 
         environment.systemPackages = [ pkgs.git ];
 
+        systemd.tmpfiles.rules = [
+          # type path mode user group age arg
+          " d    /git 0755 root root  -   -"
+        ];
+
         services.gitDaemon = {
           enable = true;
           basePath = "/git";
···
 
     with subtest("create project.git"):
         server.succeed(
-            "mkdir /git",
             "git init --bare /git/project.git",
             "touch /git/project.git/git-daemon-export-ok",
         )