All the setup/teardown/pre/post/conditions template methods
now are required to return void. This was warned with phpunit 7
and now is enforced.
At the same time, fix a few wrong function names,
provider data and param types, return statements...
Course modules can now implement the function mod_{module}_core_calendar_provide_event_action
which allows them to return an action to attach to an event.
Part of MDL-55611 epic.