Revert "Require test_helper directly in the iframe"

This reverts commit ddf5d9ead84e380c040b9dcc0d420fc0da6f62b3.
Allow configuration of loaded javascript files

The javascripts files which are loaded for each iframe before the actual spec can be configured.

Konacha.configure do |config|
  config.javascripts = %w(chai konacha/iframe test_helper)
end

This allows the same speedup but is fully compatible.