tests/unit/event_core.js: bracket prototype modifications and remove attached handlers

[gabrielschulhof]

The scope of this issue is is as follows:

1. Some prototypes are overridden in the file, but the overrides remain across modules. Instead, the overrides should be applied at setup() and removed at teardown().
2. There are many places in the file where event handlers are attached using .bind(), when .one() would suffice. There are also instances where event handlers are attached while they are not expected to fire. In those cases they should be explicitly detached.