Disable test of format string with "%n" for MSVC 8+.

Disable test of format string with "%n" for MSVC 8+.

MSVC 8 and later disables support for "%n" in printf() by default. And
although it provides a function to re-enable support for it, it doesn't seem
to work for the functions we use.

Just disable the test which results in CRT assert when using this compiler.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64920 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775