beets: fix broken aarch64 build due to missing plugin test

The `absubmit` plugin for beets has no respective plugin test that
is named after the default plugin test schema defined in `common.nix` [1],
so not explicitly declaring that the plugin has *no tests* (i.e. the
respective test list is empty), `mkPlugin` will try to disable the
non-existant test `test/test_absubmit.py`), breaking beet's build.

[1]: https://github.com/NixOS/nixpkgs/blob/c7916a507b6657f82a42741790a86e66f7783480/pkgs/tools/audio/beets/common.nix#L39

pkgs/tools/audio/beets/builtin-plugins.nix

@@ -14,6 +14,7 @@
   absubmit = {
     enable = lib.elem stdenv.hostPlatform.system essentia-extractor.meta.platforms;
     wrapperBins = [ essentia-extractor ];
+    testPaths = [ ];
   };
   acousticbrainz.propagatedBuildInputs = [ python3Packages.requests ];
   albumtypes = { };