nixos/tests: update initrd-secrets test to test secret in /run/keys
nixos/tests: update initrd-secrets test to test secret in /run/keys

Since /run/keys is a ramfs, it is not paged out and a good place to copy
secrets to. Test whether secrets with a path in /run/keys exist after initrd.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs