MDL-44492 quiz reports: make show only graded attempt work.

It needs to only affect finished attempts, and the wording needs to be
modified to make it clear that is what it does.
MDL-44492 quiz: fix hilighting of graded attempt

In various places, when showing several attempts, we try to hilight the
most significant one (the one that gave the final grade if the quiz is
set to first/last/best attempt). That was not working because the table
zebra stripes were a more specific rule.

Also, the colour was inconsistent, not nice, and not very bootstrapy, so
I changed it.
MDL-44492 Rough unit test for the report SQL.

We really ought to test more cases, but this hits one of the most tricky cases.