Allow replacement of test-case assertions on crate scale

Part of https://github.com/frondeus/test-case/issues/31.

Needs research.

`custom_test_frameworks` feature could be a solution, but it's still unstable - using it is not an option (although it would immediately solve our problems in type safe manner).

Start by reading the linked issue #31 and researching how test-case assertions could be replaced at crate scale. No files, tests, entry points, or acceptance criteria are named; completion would require defining and validating an approach without relying on the unstable custom_test_frameworks feature.

Written by the indexing model from the issue text.