symlink all shared test files

This way, the app files can be changed across all Rails versions at once.

Just FYI, I found the matching files with [fdupes](https://github.com/adrianlopezroche/fdupes), then replaced with this script:

```ruby
def symlink(dest, source)
  FileUtils.rm_rf(source)
  FileUtils.ln_s(dest, source)
end

Dir.glob('rails_*').each do |path|
  %w(mailers views).each do |subdir|
    symlink("../../shared/all/app/#{subdir}", "#{path}/app/#{subdir}")
  end
  
  unless %w(rails_30 rails_40_no_pipeline).include?(path)
    symlink("../../shared/pipeline/app/assets", "#{path}/app/assets")
  end
end
```