New test suite documentation

[Russ Allbery <rra@debian.org>]

I added a bunch of documentation to t/tests/README based on my experiences
using the new test suite over the last couple of days.  Nothing there is
set in stone; feel free to fix anything that looks wrong.  If we go with
some of the proposals in my previous message, it will change some, of
course.

I should have mentioned -- I'm willing to implement all of the ideas in my
previous message.  Most of them are fairly straightforward.

I'm going to move private/tags-never-seen to t/COVERAGE and modify the
script to generate it, and to also track what tags are only covered by the
legacy test suite so that we can see how far we've gotten towards
migrating it to a template-based system.

-- 
Russ Allbery (rra@debian.org)               <http://www.eyrie.org/~eagle/>