Add tests on release candidate when it comes out

[gdalle]

Following #374, this will be a standing issue to remind us to add tests for the upcoming release candidate when it emerges

That way we assuage @simonschoelly's concerns by detecting changes in the upcoming release (see #295 (comment)), but we don't have to deal with the inherent instability of nightly

Right now the upcoming release is still in beta, but we could already add it