[task] Port all tests to Boost.Test

[mloskot]

Currently, there is mixture of simple C++ programs used as tests with test suites/cases based on the Boost testing facilities.

I think the library should aim for unification of the tests to:

• have simpler, manageable tests organisation
• make it easier for contributors to add test cas
• receive detailed and usable failure reports from CI builds that pin-points problem, not just boolean answer pass/fail.

Timeline: some time after the new IO extension is released, near Boost.GIL 3 :

[chhenning]

boost.test or gtest?

[mloskot]

The choice of test framework is usually driven by subjective preferences, that is understandable.

From practical point, why to introduce yet another third-party library as a dependency?

I think it's reasonable to stick with the 'native' Boost tools, especially there are at least two to choose from:

1. Boost.Test (header-only or linked binary) - a heavy testing weaponry
2. Boost.Core's header-only lightweight_test.hpp, see https://www.boost.org/libs/core/doc/html/core/lightweight_test.html

I'm personally eager to go for the lightweight_test.hpp:

• small and simple
• offers all features we need
• regular main-based program
• tests are easier to debug

[mloskot]

As this is already happening, closing.

For simple single .cpp file tests, we prefer the Boost.Core.LightweightTest :-)
For more complex test suites with large number of test cases, especially if fixtures are used with, then we use Boost.Test.