stdio-common: Add tst-memstream-string for open_memstream overflow

stdio-common: Add tst-memstream-string for open_memstream overflow

This code path is exercised indirectly by some of the DNS stub
resolver tests, via their own use of xopen_memstream for constructing
strings describing result data.  The relative lack of test suite
coverage became apparent when these tests starting failing after a
printf changes uncovered bug 28949.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>