Add test of ppa command
user
```ruby
require 'spec_helper'

property[:os] = nil
set :os, :family => 'ubuntu'

describe get_command(:check_ppa_exists, 'nginx/stable') do
  it { should eq 'find /etc/apt/ -name *.list | xargs grep -o -E "deb +http://ppa.launchpad.net/nginx/stable"' }
end

describe get_command(:check_ppa_is_enabled, 'nginx/stable') do
  it { should eq 'find /etc/apt/ -name *.list | xargs grep -o -E "^deb +http://ppa.launchpad.net/nginx/stable"' }
end
```
assistant