scalafix.tests: fix eval

Without the change `tests` eval fails as:

    $ nix build --no-link -f. scalafix.tests
    error:
       … from call site
         at pkgs/development/tools/scalafix/default.nix:42:21:
           41|     passthru.tests = {
           42|       testVersion = testers.testVersion {
             |                     ^
           43|         program = "${finalAttrs.pname}";

       error: function 'testVersion' called without required argument 'package'
       at pkgs/build-support/testers/default.nix:59:5:
           58|   testVersion =
           59|     { package,
             |     ^
           60|       command ? "${package.meta.mainProgram or package.pname or package.name} --version",