Add rspec test for check_dir_count
user
```ruby
#! /usr/bin/env ruby
#
#   check_dir_count_spec
#
# DESCRIPTION:
#   rspec testing to test the functionality of check_dir_count spec
#   in particular the different combination of input options
# 
# OUTPUT:
#
# PLATFORMS:
#   Linux
#
# DEPENDENCIES:
#   rspec
#
# USAGE:
#
# NOTES:
#
# LICENSE:
#   Copyright 2015 Inny <mini.inny@gmail.com> 
#   Released under the same terms as Sensu (the MIT license); see LICENSE
#   for details.
#

require_relative '../../../plugins/files/check_dir_count'
require 'plugin_stub'

describe DirCount, 'run' do

  include_context :plugin_stub
      
  it 'return unknown if given incorrect path to directory' do 
    args = [
      '--dir',
      'incorrect/path',
      '-w',
      '10',
      '-c',
      '20'
    ]
    check_dir = DirCount.new(args)
    expected_output = "Error listing files in incorrect/path"
    expect(check_dir).to receive('unknown').with(expected_output)
    check_dir.run
  end
      
  it 'return ok if given a correct path' do  
    args = [
      '--dir',
      'spec/fixtures/plugins/files',
      '-w',
      '10',
      '-c',
      '20'
    ]
    check_dir = DirCount.new(args)
    expected_output = "spec/fixtures/plugins/files has 6 files"
    expect(check_dir).to receive('ok').with(expected_output) 
    check_dir.run
  end


  it 'return warning when number of files exceed threshold' do 
    args = [
      '--dir',
      'spec/fixtures/plugins/files',
      '-w',
      '6',
      '-c',
      '10'
    ]
    check_dir = DirCount.new(args)
    expected_output = "spec/fixtures/plugins/files has 6 files (threshold: 6)"
    expect(check_dir).to receive('warning').with(expected_output)
    check_dir.run
  end

  it 'return critical when number of files exceed threshold' do 
    args = [
      '--dir',
      'spec/fixtures/plugins/files',
      '-w',
      '1',
      '-c',
      '2'
    ]

    check_dir = DirCount.new(args)
    expected_output = "spec/fixtures/plugins/files has 6 files (threshold: 2)"
    expect(check_dir).to receive('critical').with(expected_output)
    check_dir.run
  end
end
```
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