Inventory: add assertions, allow external references

lib/inventory/tests/default.nix

@@ -1,5 +1,37 @@
-{ buildInventory, clan-core, ... }:
+{ inventory, clan-core, ... }:
+let
+  inherit (inventory) buildInventory getAllMachines;
+in
 {
+  test_get_all_used_machines = {
+    # Test that all machines are returned
+    expr = getAllMachines {
+      machines = {
+        machine_3 = {
+          tags = [ "tag_3" ];
+        };
+      };
+      services = {
+        borgbackup.instance_1 = {
+          roles.server.machines = [ "backup_server" ];
+          roles.client.machines = [
+            "client_1_machine"
+            "client_2_machine"
+          ];
+          roles.client.tags = [ "tag_3" ];
+        };
+      };
+    };
+    expected = {
+      backup_server = { };
+      client_1_machine = { };
+      client_2_machine = { };
+      machine_3 = {
+        tags = [ "tag_3" ];
+      };
+    };
+  };
+
   test_inventory_empty = {
     # Empty inventory should return an empty module
     expr = buildInventory {