TagMover doesn't eat script tags with no src

Previously, Slimmer::Processors::TagMover would only move 
script tags with a src attribute.  In order to only initialise
scripts on pages which require them, it's necessary to have
small inline scripts which might look something like this:

```
<script type="text/javascript">
	window.some_thing = new SomeThing();
</script>
```

This commit allows TagMover to identify and move script tags with
no identifiable attributes.

Also changes assert_rendered_in_template to test for any element 
matching the passed in css selector and content, instead of only
testing the content of the first element which matches the selector.