[#446] Accept either .min.js or .js in ckanext/*preview tests

We don't care if the page which one is added to the page, as long as
one of them is there.

Fixes #446.
[#446] Don't print the whole HTML whenever we get a failure

As per suggestion by @seanh in https://github.com/okfn/ckan/pull/462#issuecomment-14039804
Merge branch 'master' of github.com:okfn/ckan into 446-fixing-ckanext-preview-tests
Merge branch '446-fixing-ckanext-preview-tests'