Support title-less blocks in behat selector

I'm not really an xpath expert...

It looks to me that the block selector is looking for a <h2> of the block title.

I'm writing a test for a block (course_summary) which doesn't have a title as a h2 - but instead it has an aria-label:

<div id="inst67" class="block_course_summary  block no-header" role="complementary" data-block="course_summary" data-instanceid="67" aria-label="Course/site summary"><div class="content"><div class="block_action notitle"></div><div class="no-overflow"><p>Test Summary</p></div></div></div>

Would be cool if we could support that - since it does have an ARIA-label.