Stake table contract tests: verify correct events emitted

[sveitser]

@alysiahuggins I think many of the tests in this PR currently only assert how the contract state changes. That is fine, but what is IMO more important and less of an implementation detail is if the events with correct epochs are emitted because this is what the application will actually consume (not the state of the contract). I think we should add some asserts to check that the epochs in the emitted events evolve as expected.

Originally posted by @sveitser in #2510 (comment)