Fix failing (from time to time) test

Test was failing probably due to assets caching. Copy "wicked.css" to dummy app
on tests initialization instead of test runtime. This fixes problem.

test/functional/wicked_pdf_helper_assets_test.rb

@@ -6,9 +6,6 @@ class WickedPdfHelperAssetsTest < ActionView::TestCase
 
   if Rails::VERSION::MAJOR > 3 || (Rails::VERSION::MAJOR == 3 && Rails::VERSION::MINOR > 0)
     test 'wicked_pdf_asset_base64 returns a base64 encoded asset' do
-      source = File.new('test/fixtures/wicked.css', 'r')
-      destination = Rails.root.join('app', 'assets', 'stylesheets', 'wicked.css')
-      File.open(destination, 'w') { |f| f.write(source) }
       assert_match /data:text\/css;base64,.+/, wicked_pdf_asset_base64('wicked.css')
     end
 

test/test_helper.rb

@@ -16,3 +16,9 @@
 require 'wicked_pdf'
 
 Rails.backtrace_cleaner.remove_silencers!
+
+if (assets_dir = Rails.root.join('app/assets')) && File.directory?(assets_dir)
+  destination = assets_dir.join('stylesheets/wicked.css')
+  source = File.read('test/fixtures/wicked.css')
+  File.open(destination, 'w') { |f| f.write(source) }
+end