Build proposal: Add continuous integration to PRs, and have tests run automatically.

@sprunk suggested in Discord as I understood him that something like continuous integration could be a useful addition to the engine repository, so I am making an issue on it. I did not find any existing issues in the repository about this topic. I do not know how easy or difficult this might be to set up. It might also require build servers. However, Beyond All Reason is open source, and some companies might offer services for open source projects for continuous integration for free.

The issue proposes adding continuous integration to pull requests and running tests automatically, but it names no files, tests, build entry points, or CI service. First identify the repository's build and test commands and determine the service and build-server requirements; done means pull requests run the relevant tests automatically.

Written by the indexing model from the issue text.