tests are flaky if virtualisation.interfaces.enp0s3 is set

The following flake contains two flaky tests (oftenTimeout and oftenTimeoutSystemdInitrd) and two non-flaky tests (goodUseNetworkd and goodAnotherName). When run multiple times, the two flaky tests often timeout.

Comparing oftenTimeout and goodAnotherName, we find that enp0s3 is flaky but enp1s3 is not, which is quite unexpected and interesting.

# flake.nix
{
  description = "Minimal reproducible example of flaky virtualisation.interfaces.enp0s3";
  # 2026-02-05
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/00c21e4c93d963c50d4c0c89bfa84ed6e0694df2";
  outputs =
    { nixpkgs, self }:
    let
      system = "x86_64-linux";
      pkgs = nixpkgs.legacyPackages.${system};
      mkTest =
        interface: useNetworkd: useSystemdInitrd:
        pkgs.testers.runNixOSTest {
          name = "ifname";
          nodes.machine = {
            networking.useNetworkd = useNetworkd;
            boot.initrd.systemd.enable = useSystemdInitrd;
            virtualisation.interfaces.${interface} = {
              vlan = 1;
              assignIP = true;
            };
          };
          testScript = ''machine.wait_for_unit("multi-user.target", timeout=60)'';
        };
    in
    {
      packages.${system} = {
        default = self.packages.${system}.oftenTimeout;
        oftenTimeout = mkTest "enp0s3" false false;
        oftenTimeoutSystemdInitrd = mkTest "enp0s3" false true;
        goodUseNetworkd = mkTest "enp0s3" true false;
        goodAnotherName = mkTest "enp1s3" false false;
      };
    };
}

Start with the flake.nix reproducer and run the four packages repeatedly, comparing oftenTimeout and oftenTimeoutSystemdInitrd with goodUseNetworkd and goodAnotherName. Trace the enp0s3 interface setup and test timeouts; done means the flaky tests no longer time out reliably under the reproducer.