Add test suite for color_convert and default_color_converter

We need to start a detailed test cases coverage for color conversion algorithms:

- Add `test/core/pixel/color_convert.cpp`
- Use Boost.Test to define test cases using `BOOST_AUTO_TEST_CASE` or `BOOST_AUTO_TEST_CASE_TEMPLATE`
- Add test cases for each pre-defined specialisations of converters
- Covert byte-based as well as bit-aligned and packed pixel types

---

It's a good idea to exercise the `color_convert` and `default_color_converter` using available interfaces, for example

```cpp
gil::rgb8_pixel_t red(255, 0, 0);
... red2;

// three equivalent conversions
gil::color_convert(red1, red2);
gil::default_color_converter()(red1, red2);
red2 = gil::color_convert_deref_fn()(red1);
```