Try to achieve 100% test code coverage to find accidentally missed ones.
Especially the WinAPI tests had some wrongly uncovered lines at function
start/end which needed to be excluded.
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.
