* This translation unit includes `gtest.h` in which using something like `strncpy`

Suppress MSVC warnings from `gtest.h` included.

Warning sample:
fmt-master\test\gtest\gtest.h(2873,10): warning: 'strncpy' is deprecated: This function or variable may be unsafe. Consider using strncpy_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details. [-Wdeprecated-declarations]

* Moving `test_count_digits` to format-impl-test then count_digits<4>(internal::uintptr_t) shall not be exported

* retaining _CRT_SECURE_NO_WARNINGS always defined should be no harms.