libstdc++-v3 testsuite patch

[Paolo Carlini]

Hi Chris,

> A hopefully entirely non-exciting libstdc++-v3 testsuite patch, which
> adds another couple of functions, extends some previous ones to
> improve coverage, and also fixes a bug in search_n's test. 

I see that you posted this patch a few days ago already, and nobody
replied, therefore I'm going to apply it later today, in the v7-branch
only, for now (but as soon as the testsuite improvements stabilize a
bit, will be definitely ported to mainline)

Paolo.