Add test cases for ProcessWatcher::ProcessMonitor.
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```ruby
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require File.expand_path(File.join(File.dirname(__FILE__), '..', 'spec_helper'))

describe ProcessWatcher::ProcessMonitor do

  before(:each) do
    @monitor = ProcessWatcher::ProcessMonitor.new
  end

  after(:each) do
    @monitor.cleanup
  end

  it 'should launch and watch well-behaved processes' do
    ruby = "puts 42; exit 42"
    times = 0
    pid = @monitor.spawn('ruby', '-e', ruby) do |hash|
      case times
      when 0 then hash.should == {:output => "42\n"}
      when 1 then hash[:exit_code].should == 42
        hash[:exit_status].should_not be_nil
      else fail "Shouldn't see more than two ticks"
      end
      times += 1
    end
    @monitor.cleanup
    lambda { Process.kill(0, pid) }.should raise_exception(Errno::ESRCH)
  end

  it 'should launch processes where we don\'t care about output' do
    ruby = "puts 42; exit 42"
    times = 0
    pid = @monitor.spawn('ruby', '-e', ruby)
    @monitor.cleanup
    lambda { Process.kill(0, pid) }.should raise_exception(Errno::ESRCH)
  end
end
```
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