PR for addressing test related notes

Related: https://github.com/edx-solutions/xblock-drag-and-drop-v2/pull/19

Work in progress
The fix introduced one really subtle problem: zone ids are the same as their titles, so all the HTML put up in zone title gets into zone id as well. I decided that it wouldn't hurt having same set of tests that check "attempts" both for plain and html zone titles. Also, it was simper to implement. :)