[Truffle] JT: Let :files be the default for MSpec.

* So we also run :library on `jt test`.

Author: eregon

tool/jt.rb

@@ -240,7 +240,6 @@ def test(*args)
       args.shift
       options += %w[--excl-tag slow]
     end
-    args = [':language', ':core'] if args.empty?
     mspec 'run', *options, *args
   end