Use wxUSING_VC_CRT_IO for MSVC CRT-specific test results.

Use wxUSING_VC_CRT_IO for MSVC CRT-specific test results.

Move USING_VC_CRT into testprec.h to allow its reuse in other files and rename
it to wxUSING_VC_CRT_IO as it only checks whether we're using MSVC STDIO
implementation and could be false even when we are otherwise using MSVC CRT.

Use this symbol for the tests whose result depends on the concrete version of
the CRT we use.

This fixes StringTestCase::FromDouble() failure under MinGW.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65745 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775