initrd-secrets test does not test that a secret actually ends up in the initrd

[nikstur]

The test executes a grept that is supposed to fail if it does not find the secret in the initrd or if the contents are not as expected with boot.initrd.preDeviceCommands. However, even if this grep fails, the system comes up normally and the tests succeeds.

See: https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/initrd-secrets.nix for a better way to test this.