This refactor of integration test logic is to boost its
reusability. Moving some functionality to attributes &
customization classes means I can reuse elsewhere without
duplication.
Temporal activities no longer have a generalised extension method
for registering them with a service collecton. It is still static and has
the same rough shape as an extension method. This way it is less
visible and less likely to be called by accident.
Also in-line with changes in this branch, things have been renamed
Timers → Temporal
