tests: add test to samples - infrastructure

This is the first stab at introducing DOM-based unit test. There are only tests for 2 samples now, but opening this PR early on to:
* show which config changes would be needed
* discuss one of the not-so-nice effect connected with the fact that we've got `.render('output')` in each and every sample; this generates a warning in each test.

I had a short chat about this with @b-laporte today and we though of overriding a logger (appender) but in fact #tester already does it ... only a bit too late. So we need to find a better way of having samples rendered _and_ avoiding the call to `.render('output')` under tests. Any ideas?