GitHub Actions: Test project exporting on CI

This allows finding issues in headless project export early on,
including when exporting for a dedicated server.

We also use this opportunity to check whether the audiovisual output
between the project being run from its files and the exported PCK
matches (it should always be a perfect match, assuming the same GPU
is used for both runs). This can be used to catch audiovisual
discrepancies, which could indicate a bug in the export process.