How do I use parallel testing in Ruby 1.9.3?

(Cross-posting from
http://stackoverflow.com/questions/10445786/ruby-1-9-3-doesnt-run-any-unit-tests-when-run-in-parallel
)

Given the following file

# Modified from
https://gist.github.com/1334056/121396125ca4dd7ee1bb81536cce0754700f20fb
# replication.rb
require 'test/unit'
class TestTest < Test::Unit::TestCase
  def test_one
    assert true
  end

  def test_two
    assert true
  end
end

and Ruby 1.9.3

$ /usr/bin/ruby1.9.1 --version
ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-linux]

$ /usr/bin/ruby1.9.1 replication.rb --help
Usage: replication [options]
minitest options:
    -h, --help                       Display this help.
    -s, --seed SEED                  Sets random seed
    -v, --verbose                    Verbose. Show progress processing files.
    -n, --name PATTERN               Filter test names on pattern.
        --jobs-status [TYPE]         Show status of jobs every file;
Disabled when --jobs isn't specified.
    -j, --jobs N                     Allow run tests with N jobs at once
        --no-retry                   Don't retry running testcase when
--jobs specified
        --ruby VAL                   Path to ruby; It'll have used at -j option
    -q, --hide-skip                  Hide skipped tests
    -b, --basedir=DIR                Base directory of test suites.
    -x, --exclude PATTERN            Exclude test files on pattern.
    -Idirectory                      Add library load path
        --[no-]gc-stress             Set GC.stress as true

I'm able to run tests non-parallelized:

$ /usr/bin/ruby1.9.1 replication.rb
Run options:

# Running tests:

..

Finished tests in 0.000458s, 4368.4051 tests/s, 4368.4051 assertions/s.

2 tests, 2 assertions, 0 failures, 0 errors, 0 skips

But not in parallel:

$ /usr/bin/ruby1.9.1 replication.rb --jobs=2 --ruby /usr/bin/ruby1.9.1
Run options: --jobs=2 --ruby /usr/bin/ruby1.9.1

# Running tests:


Retrying...





Finished tests in 0.037315s, 0.0000 tests/s, 0.0000 assertions/s.

0 tests, 0 assertions, 0 failures, 0 errors, 0 skips

I've also found this to be the case on both Ubuntu (12.04) and OS X
(snow leopard).

Do I need to be using rake in order to do parallel testing? Or have I
done something else wrong?

Thanks,

Andrew Grimm