Include tests and useful/related test files in sdist

[koobs]

This is useful for downstream users, including OS packagers to QA/test if the package works correctly without requiring fetching the sources from the repository.

Identified by: FreeBSD Ports Bug 242694

Closes: #55

[koobs]

I note this is a duplicate of #57, apologies

[patrys]

@koobs I think the same point stands, unless we make it a package, the extra files could potentially collide with files shipped by another package.