tests : add _CRT_SECURE_NO_WARNINGS for WIN32 (#8231)

This commit adds the compile definition `_CRT_SECURE_NO_WARNINGS`
to the root cmake subproject.

The motivation for this is that currently the following warnings are
displayed when compiling the tests and common cmake subprojects:
```console
test-llama-grammar.cpp
C:\llama.cpp\src\.\llama.cpp(1406,77): warning C4996: 'strerror':
This function or variable may be unsafe. Consider using strerror_s
instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See
online help for details.
[C:\llama.cpp\build\tests\test-llama-grammar.vcxproj]
...
```

This compile definition is currently set for the `src` subproject
and this change moves into the root cmake project so that it is applied
to all cmake subprojects.