inventory test framework: init

Add nixos test module checks/lib/test-inventory-nix

accepts new option `inventory` and creates machines accordingly with correct imports

checks/flake-module.nix

@@ -41,6 +41,7 @@ in
             borgbackup = import ./borgbackup nixosTestArgs;
             matrix-synapse = import ./matrix-synapse nixosTestArgs;
             mumble = import ./mumble nixosTestArgs;
+            dummy-inventory-test = import ./dummy-inventory-test nixosTestArgs;
             data-mesher = import ./data-mesher nixosTestArgs;
             syncthing = import ./syncthing nixosTestArgs;
             zt-tcp-relay = import ./zt-tcp-relay nixosTestArgs;