python3.pkgs.beetcamp: add passthru.tests.beets-with-beetcamp

See: https://github.com/NixOS/nixpkgs/pull/473678#discussion_r2644975474

pkgs/development/python-modules/beetcamp/default.nix

@@ -13,6 +13,7 @@
   filelock,
   writableTmpDirAsHomeHook,
   nix-update-script,
+  beetcamp ? null, # For `passthru.tests`.
 }:
 
 let
@@ -58,7 +59,19 @@ buildPythonPackage {
     "test_get_html"
   ];
 
-  passthru.updateScript = nix-update-script { };
+  passthru = {
+    updateScript = nix-update-script { };
+    tests = {
+      beets-with-beetcamp = beets.override {
+        pluginOverrides = {
+          beetcamp = {
+            enable = true;
+            propagatedBuildInputs = [ beetcamp ];
+          };
+        };
+      };
+    };
+  };
 
   meta = {
     description = "Bandcamp autotagger source for beets (http://beets.io)";