ci: split debian-build-testing?

This job takes forever relative to other build jobs. 10 minutes to build, then it starts running tests?

A problem I've seen more than once is that the tests fail/hang: https://gitlab.freedesktop.org/mesa/mesa/-/jobs/47046796

Not only do they fail, they fail to complete. This leads to the job sitting for however long until somebody (maybe) notices, and then the whole thing has to be retried. It seems like it'd be much smarter to split the build job from the tests so that the test job can be rerun more reliably without hitting the timeout, as rerunning a 5-6 minute test job is much more feasible than some 15-20 minute build+test job.