Modules-specific tests can be written under respective module folders.
Each module should have "tests" folder created with the tests implemented
as `doctest` headers, so they can be collected by the buildsystem and
included directly in `tests/test_main.cpp` to be compiled.
Allows switching `tests=yes`/`no` and rebuilding only tests and main,
instead of the whole engine.
Co-authored-by: Andrii Doroshenko (Xrayez) <xrayez@gmail.com>
