* It's reassuring to see that the changes I independently arrived at
in e9d05cc are almost identical to
those @kou made in #89.
* This commit brings the code more into line with what @kou recommended,
although I haven't extracted all the private methods that he did -
they seem like overkill to me.
* I'm still curious to know whether #problem_occurred and #add_failure
are part of the Test::Unit public API...?