clanServices: Move mycelium test to appropriate location

Move the mycelium clanService test to the mycelium clan service.
The tests should live close to the definition of the service, so that
debugging and discoverability is better.

clanServices/mycelium/flake-module.nix

@@ -1,6 +1,17 @@
-{ lib, ... }:
+{ lib, self, ... }:
 {
   clan.modules = {
     mycelium = lib.modules.importApply ./default.nix { };
   };
+  perSystem =
+    { pkgs, ... }:
+    {
+      checks = lib.optionalAttrs (pkgs.stdenv.isLinux) {
+        mycelium = import ./tests/vm/default.nix {
+          inherit pkgs;
+          clan-core = self;
+          nixosLib = import (self.inputs.nixpkgs + "/nixos/lib") { };
+        };
+      };
+    };
 }