Fix tests for MSVC

warnings-as-errors caused the simple_seg_storage and threading tests to fail on all Visual C++ compilers. This define suppresses the warnings about unchecked iterators, which I don't think are solvable in a portable way.
Merge pull request #3 from MarcelRaad/patch-1

Fix tests for MSVC

I don't have access to Windows machine but this patch looks okay.