How to run multiple Rails unit tests at once

I often run the various test groups like:

rake test:units
rake test:functionals

I also like to run individual test files or individual tests:

ruby -Itest test/unit/file_test.rb
ruby -Itest test/unit/file_test.rb -n '/some context Im working on/'

There's also:

rake test TEST=test/unit/file_test.rb

And I've even created custom groupings in my Rakefile:

  Rake::TestTask.new(:ps3) do |t|
    t.libs << 'test'
    t.verbose = true
    t.test_files = FileList["test/unit/**/ps3_*_test.rb", "test/functional/services/ps3/*_test.rb"]
  end

What I haven't figured out yet is how to run multiple ad-hoc tests at the command line. In other words, how can I inject test_files into the rake task. Something like:

rake test TEST=test/unit/file_test.rb,test/functional/files_controller_test.rb

Then I could run a shell function taking arbitrary parameters and run the fast ruby -Itest single test, or a rake task if there's more than one file.