ability to check that block content contains specific blocks in arbitrary places

[kapouer]

Currently it's impossible to validate this:

• create a form_consent (which has specific behaviors upon submit)
• expect it to contain "yes" and "no" inputs
• validate upon save

or this:

• create a fieldset-list
• the template is expected to contain "add" and "del" buttons

However the place of the expected blocks is arbitrary in those examples.