When copying properties from one stream to another the old stream only
gets a `erase_event` but may or may not have the value reset/zeroed.
So do that in all cases even though it is redundant on stream destruction.
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`
To support e.g. recent Cygwins where relative paths to the current file are not automatically resolved
move the sources into a `boost/locale` subfolder and add `src` to the private include paths.
This then allows to include those files as `"boost/locale/foo/bar.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.
