Add test for pixel_reference_is_mutable metafunction.
From the legacy tests
- port value_core and reference_core fixtures, see
https://lists.boost.org/boost-gil/2019/02/0138.php
- port representative pixel types and verify with tests of some
metafunctions.
Clean up test names for CTest in the CMake configuration.
Disable some GCC/clang warnings in tests to avoid CI build
termination due to too long logs.
