Tests were written with a very flat module hierarchy which made them
a bit confusing to navigate. This commit moves everything under a common
`Test.*` hierarchy.

Alas I (Alfredo) had to split the runners for tasty and hspec because in
tasty we cannot control explicitly the level of parallelism, but for the
DB stuff we need sequential tests, whereas for the Jobs tests we need
parallelism, or rather the executable to be compiled with `-threaded`.

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.