Add tests to repo

[xvxx]

There is an out-of-date test suite at: https://github.com/Lartu/ldpltest

I think it would be really nice if the tests were included and you could just run make test to see if you broke anything, without having to mess with another repo:

• Add tests into the main repo
• Mention them in the README
• Mention them in the CONTRIBUTING guide
• Require tests for all new statements and features
• Run tests using GitHub Actions after every commit
• Add make test or some other way to easily run the tests

I am not sure why they're not part of the repo, honestly, so I'm not really sure what case to make. Right now we are making it harder for people to contribute by requiring them to do work to get the tests going, when in many other software projects the tests are just included in the repo directly.

If they really need to be kept in a separate repo, it would be nice if they were included as a git submodule and there was an automated way to clone and test them.