Fix some GC related tests to work with MSVC

Using alloca instead of memset we can convince even MSVC to allocate memory on the
stack, to overwrite any previous dead V4 pointers.

Change-Id: Ic01bebfc6368e9c3ce1f6155a0f0ea206b90764c
Reviewed-by: Lars Knoll <lars.knoll@digia.com>