MemoryInputStream test: Small improvements
MemoryInputStream test: Small improvements

* tests/giomm_memoryinputstream/main.cc: Slightly better test of the
deletion of the data. More output in case of failure.

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,johne53/MB3Glibmm,GNOME/glibmm,GNOME/glibmm,johne53/MB3Glibmm,johne53/MB3Glibmm,johne53/MB3Glibmm