Improve EventSpyExt utils

[immrsd]

For now, assert_event_emitted checks that the next event is the expected one and then increments the offset variable. It leads to several problems:

• the expected event must be the next one, otherwise the assertion fails
• the order of events matters
• if there's an event from another contract, assertions start to fail

[virajbhartiya]

Hey @ericnordelo can i work on this?

[ericnordelo]

Hey @virajbhartiya. Thanks for your interest in contributing! We plan to take this particular issue since it involves careful design around the API of the solution. I recommend you to check the list of good first issues to start contributing to the library.