Kill process launched via thread

I have a headless application that uses sockets for communication. When launched, it remains active until sent a message telling it to quit (or it crashes, or is killed).

When unit testing this application using Ruby, I need to launch the process, interact with it (via sockets), and then kill it.

I thought I could do this using this pattern:

class TestServer < MiniTest::Unit::TestCase
  def setup
    @thread = Thread.new{ `#{MY_COMMAND}` }
  end

  def test_aaa
    # my test code
  end

  def teardown
    @thread.kill if @thread
  end
end

However, that teardown code kills the thread but does not kill the process launched by it.

How can I launch the process in a way that:

1. Allows it to run in the background (immediately returns control to my Ruby test harness)
2. Allows me to force kill the process later on, if need be.

I happen to be developing on OS X, but if possible I'd appreciate a generic solution that works across all OS where Ruby runs.