Disable THOR_DEBUG while running generators

Bundler 1.5 enables THOR_DEBUG so that it doesn't fail silently but this
breaks the railties tests as we can't capture the error output. It will
be fixed in Bundler 1.5.2 but until then disable THOR_DEBUG whilst we are
running generators in the railties tests.

railties/lib/rails/generators/testing/behaviour.rb

@@ -61,9 +61,11 @@ def destination(path)
         # You can provide a configuration hash as second argument. This method returns the output
         # printed by the generator.
         def run_generator(args=self.default_arguments, config={})
-          capture(:stdout) do
-            args += ['--skip-bundle'] unless args.include? '--dev'
-            self.generator_class.start(args, config.reverse_merge(destination_root: destination_root))
+          without_thor_debug do
+            capture(:stdout) do
+              args += ['--skip-bundle'] unless args.include? '--dev'
+              self.generator_class.start(args, config.reverse_merge(destination_root: destination_root))
+            end
           end
         end
 
@@ -100,6 +102,14 @@ def migration_file_name(relative) # :nodoc:
             dirname, file_name = File.dirname(absolute), File.basename(absolute).sub(/\.rb$/, '')
             Dir.glob("#{dirname}/[0-9]*_*.rb").grep(/\d+_#{file_name}.rb$/).first
           end
+
+          # TODO: remove this once Bundler 1.5.2 is released
+          def without_thor_debug # :nodoc:
+            thor_debug, ENV['THOR_DEBUG'] = ENV['THOR_DEBUG'], nil
+            yield
+          ensure
+            ENV['THOR_DEBUG'] = thor_debug
+          end
       end
     end
   end