Adds tests for panel structure

Refs #51.

Approached this one by selecting the inner (optional) class and test that its parent has the required *parent* class. (e.g. select for `.panel-title` and check that it has `.panel-heading`).
Hm. I should explicitly add a check for `.panel-collapse` as a valid parent of `.panel-body`. I'll get to it when I'm in front of a proper computer in the morning, Need to read through the panels.less file a bit more.