clan tests: allow testing CLI interactions

This is an improvement of the clanTest nixos vm test module.

The module now has a new option clan.test.fromFlake that allows to specify a flake.nix as the source for the test clan instead of specifying clan.XXX options.

This in turn allows accessing the `flake.nix` inside the test driver allowing to use the clan cli on it

checks/dummy-inventory-test-from-flake/legacy-module/roles/admin.nix

@@ -0,0 +1,5 @@
+{
+  imports = [
+    ../shared.nix
+  ];
+}

checks/dummy-inventory-test-from-flake/legacy-module/roles/peer.nix

@@ -0,0 +1,5 @@
+{
+  imports = [
+    ../shared.nix
+  ];
+}