We need to fix them properly as part of #198.

This commit partially rewrites the `testPrios` queue test, in such a way
that it's now easier to inspect the final schedule and the occurrence of events.
Albeit not perfect, it should be less sensitive to random failures,
like the ones we have been observing on CI.

This big commit improves the jobs API in a way that now we can
completely abstract away over a JobLog.