Use a custom tag when substituting components in test

Using a script tag caused a few issues when used in an app:
* it isn't "visible" to Capybara, requiring overriding default behaviour for
  Capybara matchers to work
* this meant that the app had to have more knowledge than it should about
  Slimmer
* Slimmer automatically moves script tags to the end of the body tag, making it
  impossible to test where the component was being included
* whilst it was less likely to be confused for real content, it still could be
  confused for a real script tag
Add a regression test for SharedTemplates test helper
Reuse the govuk_component substitute tag name

This should make it harder for the test helper to get inadvertantly broken.
Merge pull request #106 from alphagov/improve_govuk_components_test_behaviour

Improve govuk components test behaviour