clanServices: Move deltachat test to appropriate location

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

clanServices/deltachat/flake-module.nix

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