Negative tests for the libstdc++ debug mode?

[Doug Gregor]

My previous patch only supplied tests that check the invalidation rules 
for containers. While this gives reasonably good coverage, I should 
probably supply some of the negative tests I use. However, I don't 
think we want a huge number of these, because they are quite repetitive 
and one can easily generate lots of them. So the question is: how many 
is too many? Would everyone be satisfied with one negative debug test 
per container, exercising the different error messages? I use about 50 
of these locally, but they very rarely find anything that isn't found 
by running the libstdc++ testsuite with debug mode.

	Doug