This commit fixes several minor issues and bugs in `mesa.time` and `mesa.model` related to event scheduling and run control.

1. It moves the relevant tests from `mesa.experimental.test_devs` into a new folder (time) and test_events. This ensures the test structure mimics the layout of the repo.
2. It improves code coverage for Schedule. Specifically, it adds additional tests for the post-init validation.
3. It adds a check that `interval` in the EventGenerator raises an exception if it is negative (in case of callable) and adds a test for this.
4. It removes the return from EventGenerator.stop. The idea of stop is that it stops and deposes the generator. There is no need for a return.
5. The eventlist `__len__` and `__contains__` now ignore canceled events. This is also explicitly tested.