Define `_SCL_SECURE_NO_WARNINGS` for building the library to avoid "Call to 'std::copy' with parameters that may be unsafe" in `src\boost\locale\std\converter.cpp`
This also allows to remove the multiple suppressions of C4996
Move the skip condition into `test_main` to avoid missing compile failures due to wrong/missing includes in the headers and to improve formatting due to the removed indentation level of the PP macros
Gets rid of the `test_` prefix in the filenames and avoids confusing those with the actual tests especially by renaming the test framework file from `test_locale.hpp` to `unit_test.hpp`
Use a common `main` function calling into custom `test_main` functions and
move all common code into the former.
Also introduce an RAII class for a `locale_t` to avoid leaks and reduce
the code further.
Add some LCOV annotations for lines not expected to be reached.
For consistency the following rules are used:
- public includes (include folder) either include any <boost/locale/*>
first. At least <boost/locale/config.hpp>
- The first include in a cpp file is the corresponding header if any
- Then <boost/locale/*>, <boost/*>, <*>, "*" in this order
- Warning suppression pragmas come last where possible
- Use the C++ headers where available (i.e. <cstring> not <string.h>)
