Added generated unit test for generator generator.

## Before this patch

no test was generated for ```rails g generator awesome```.

## With this patch

For ```rails g generator awesome``` is generated:

```ruby
# test/lib/generators/awesome_generator_test.rb
require 'test_helper'
require 'generators/awesome/awesome_generator'

class AwesomeGeneratorTest < Rails::Generators::TestCase
  tests AwesomeGenerator
  destination Rails.root.join('tmp/generators')
  setup :prepare_destination

  # test "generator runs without errors" do
  #   assert_nothing_raised do
  #     run_generator ["arguments"]
  #   end
  # end
end
```

## Changes

* I created new ```test:generators``` rake task and added it into ```test:all``` rake task.
* I added ```require 'rails/generators/test_case'``` into ```railties/lib/rails/test_help.rb```. I'm not sure how big dependency is that. Maybe we could require it per test file.

## Problems

* I couldn't find any hint how to test generators in [rails testing guides](http://guides.rubyonrails.org/testing.html). Maybe some small entry will be fine. What do you think?

/cc @steveklabnik