llm.tests: fix the eval

Without the change `llm.tests` fails the eval on `master` as:

    $ nix build --no-link -f. llm.tests
    error:
       … in the right operand of the update (//) operator
         at lib/attrsets.nix:1619:60:
         1618|           # The invariant is satisfied because each half will have at least 1 element
         1619|           binaryMerge start (start + (end - start) / 2) // binaryMerge (start + (end - start) / 2) end
             |                                                            ^
         1620|         else

       … in the left operand of the update (//) operator
         at lib/attrsets.nix:1619:11:
         1618|           # The invariant is satisfied because each half will have at least 1 element
         1619|           binaryMerge start (start + (end - start) / 2) // binaryMerge (start + (end - start) / 2) end
             |           ^
         1620|         else

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: attribute 'tests' missing
       at pkgs/development/python-modules/llm/default.nix:262:46:
          261|       # include tests for all the plugins
          262|       tests = lib.mergeAttrsList (map (name: python.pkgs.${name}.tests) withPluginsArgNames);
             |                                              ^
          263|     };

It happens because `python.pkgs.llm-lmstudio.tests` attribute does not
exist. THe change adds a fallback for packages without tests attributes.